The brief was every architect's dream: make an iconic building. Easier said than done and not helped by our penchant for applying the word to everything from buzzy bees, films, rock bands, retailers, sheep, magazines, meat pies, baches, even netball shooters. It's hard to take the term seriously, let alone know what it means.

When applied to architecture, iconic tends to indicate having an eternal "wow" factor. Hence the Sydney Opera House, Frank Gehry's Guggenheim Museum in Bilbao and Daniel Libeskind's Jewish Museum in Berlin.

It's not something you expect to find turning right at Otorohanga. Yet, after negotiating a jumble of signs anticipating arrival, and finally rounding a corner of Waitomo Caves Rd, the first encounter of the Waitomo Glowworm Caves Visitor Centre does indeed elicit a "wow". It may not be on the same scale or budget of the well-known icons but, at the very least, a visitor will know they have arrived.

The entrance to the $13 million building is under an arching translucent canopy with an arresting asymmetry, as though one end has been slashed off mid-air, mid-span. Last night the Visitor Centre won the New Zealand Architecture Medal, the building of the year award from the New Zealand Institute of Architects.

In architecturespeak the form is known as a gridshell - a lattice, net-like structure that gets its strength from its double curvature. For the local Ruapuha Uekaha hapu, which owns the caves in partnership with the Department of Conservation, the woven tunnel was instantly recognisable as like a hinaki or Maori eel trap.

Except that what's being trapped here are tourists, corralled at the beginning of an architectural journey amid native bush. To the right, in the mouth of the hinaki, is the resurrected, restored and revitalised pou - the carved totara pole of Tane Mahuta, god of the forest.

It was carved in 1986-1987 by Tutunui Te Kanawa to mark the centenary of the Waitomo Glowworm Caves' first recorded exploration, and the master carver was also involved in its restoration.

"The placement of that big pou was quite important to us. At the time I described that as the most photographed Maori carving in the world because it probably is," says chairman of the Ruapuha Uekaha Hapu Peter Douglas. "Most tourists who come through there will get a photograph by that. We wanted to maintain the prominence that it had before."

Among clamouring billboards advertising attractions and in front of a palisade of redwood planks punctuated by ticket office booths, the pou glistens with newly inset paua representing titiwai (glowworm) feeding lines, and with parts of the carving brightly painted. "Originally the whole thing was red," says Robert Tahi, who has worked at the caves for 30 years. "As we took it down to move it we decided to paint it again and decided to paint the birds and animals with colours to give more appeal."

To the left, bunker-like and buried in the bank of the sloping site, is another essential element in a tourist pitstop - the toilets. An embarrassed male tourist pops out of the women's entrance, having misread the "Wahine", "Tane" signs.

As the group of tourists departs the waiting area and begins down the path, the grand design of the gridshell becomes apparent. It serves as an open-ended shelter leading to the main attraction - the cave with its spectacular cathedral space where tourists for generations have tested its acoustics in song, and its silent, eerie glowworm grotto boat ride.

Leaving the boat at the base of the cave by the stream below, tourists then follow the path back up through the bush and are once again under the soaring canopy - on a lower deck with various amenities including restaurant, conference rooms and giftshop.

"It was part of the brief that we wanted an iconic building," says chief information officer Quinton Hall of Tourism Holdings, which owns the building and runs the Waitomo Caves operation under lease. "We felt that the cave is such an iconic cave in New Zealand history with vast numbers of New Zealanders going there as children and taking their children back there, that it justified a statement."

But when Tourism Holdings was presented with the gridshell concept by architect Christopher Kelly of Wellington's Architecture Workshop, it baulked. "Although we were quite taken with it, there were definitely some unknowns about it and it's definitely had some risks attached to it," says Hall.

As if the challenge of building a gridshell wasn't enough, Kelley wanted to build the canopy in a woven laminated radiata pine lattice and cover it in inflated plastic pillows made of ethylene tetrafluoroethylene (ETFE).

Kelly's Architecture Workshop had been involved in the feasibility phase of the project and had presented a number of schemes - including an even more radical idea of a tunnel over the river - and thought it had developed a good rapport with the client. But much to the architects' chagrin, the Tourism Holdings Board decided it was prudent to investigate other options and engaged three other architectural firms to come up with concept designs.

"We thought, 'bugger'," says Kelly. "We had done all this work and we knew the site. Then we thought it would show if we were any good. It was a fair enough test."

Hall says despite the risks the company came back to Kelly's design because it was so compelling.

"We liked the fact that it was within the environment - that you were kind of inside and outside and connected to the place. Given we were going to invest some significant money, we decided to do something that was reasonably brave."

The project had come about as a result of a fire in December 2005 which destroyed the existing visitor centre.

"It was devastating," recalls Department of Conservation Waikato conservator Greg Martin. "I looked in disbelief because there was just smouldering remnants left."

Martin is chairperson of the Waitomo Cave management committee - a partnership between Ruapuha Uekaha Hapu Trust and the Department formed after the treaty settlement of 1990 which returned the cave's land to the hapu.

The fire also destroyed most of the towering redwood trees planted there in 1905. Martin says after much soul searching it was decided to remove the few remaining undamaged redwoods, returning the site to indigenous trees only and using the timber in the new building.

The committee's role was to advise on what cultural elements should be included in the building. Initially the plan was to include a 50-seat virtual reality style theatre that would tell a "journey to the centre of the earth" story incorporating the Maori creation myth.

In the end the idea didn't stack up and was dropped in favour of a smaller audio-visual theatre which shows a video of local Maori and Pakeha talking about what the place means to them. Martin was relieved.

"My worry about that first concept was that it was rather theme park-ish. I did worry about the move away from the natural experience which after 100 years of tourism is what people go there for."

Meanwhile, Architecture Workshop had been doing much to allay client fears. It had the structure peer reviewed by London engineering consultancy Happold Structural Engineers to show a pine gridshell - not tried before - could be built.

Kelly says as well as promoting pine's plantation-grown sustainability, he wanted to show that while pine is inherently a weak timber, if laminated it develops considerable strength and anything is possible.

Credit for the engineering goes to Alistair Cattanach at Dunning Thornton Consultants who worked closely with Architecture Workshop throughout the project.

Such close collaboration is a hallmark of Kelly's practice - something he gained from his time working under renowned Italian architect Renzo Piano, which included being part of the design team for Kansai International Airport's airfoil roof.

It was an experience that gave Kelly confidence to tackle the gridshell which he saw as an efficient way to enclose a large volume.

"I knew how to play with those geometries and how to work with that structure," he says. "But it is really a conversation between the engineer and the architect and I was comfortable having that conversation. What we are talking about is a poetic interpretation of the minimal amount of structure needed."

Although Architecture Workshop was, in 2006, trailblazing with ETFE here, Tourism Holdings was reassured the material had plenty of international precedents - including the Beijing Olympics National Aquatics Centre, also known as the Water Cube.

The fluorocarbon-based polymer had been around since the 1970s when DuPont invented it for insulation in the aeronautics industry.

But it wasn't until the early 80s that its architectural qualities began to be realised. The resin can be spun into a thin, surprisingly durable, film - like a sturdier version of plastic cling-wrap - and it can be used in sheets or inflated into pillows.

The downside is that the EFTE can be very expensive - especially if it's patterned into individual cushions that fit between the lattice. By making long sausage-like pillows slung between parallel ribs of the gridshell running in one direction, costs were significantly reduced. Further cost savings came through foregoing the "fritting" of the plastic with a printed pattern to reduce solar gain.

The latter decision produced an unintended consequence - architecturespeak for "mistake". Even though the canopy is open at both ends, at the height of summer with no breeze it can get quite hot under the translucent roof - on occasion as high as 43C.

What was surprising, says Hall, was that everyone expected the warm air to rise and to move out from either end of the tunnel, but somehow, in part due to EFTE's thermal properties, the hot air, on some days, stays put. As a workaround Tourism Holdings has extra umbrellas and fans on hot days on the lower deck, and is investigating adding further as-needed shading screens.

Another unintended consequence is driving rain and wind on some winter days at the bush-end of the gridshell - also unexpected because it was thought the bush would provide shelter. To deal with the problem, temporary clear screens are likely to be added.

Is he annoyed by the unintended effects? "I'd have liked it to be completely perfect," says Hall.

"Given the nature of those two issues and the testing that we did, it's one of those things that I don't think anyone would have been able to pick up - so you can't really be too mad."

Hall says the building has already delivered more than expected and holds possibilities for expanded use as a conference and events facility.

The test will be whether it helps increase tourist numbers - currently around 300,000 a year - especially in repeat visits from the domestic market which accounts for just 20 per cent of visitors. Already there are signs it may be working. A family with two children makes its way down the path. "We have to go this way," says the younger child. "Do you remember this from before? Are you excited?" says the mother. "Yes, I am. I've dreamed about it."