Transport Agency designers have agreed with opponents of the $1.75 billion Waterview motorways project that the height of proposed towers for venting vehicle emissions from tunnels should be reduced.

Their agreement, from a meeting of expert witnesses, will be reported to a Government-appointed board of inquiry after the opening today of a marathon hearing on the country's largest transport undertaking.

Three consultants hired by the agency have accepted it should consider reducing a 25m tower at Waterview to a set of three emission stacks of 15m to 17m to break up the visual impact, and that a 27m tower above Alan Wood Reserve in Owairaka needs "minimisation".

Judge Laurie Newhook of the Environment Court will chair the five-member board considering 54 resource consents as well as land designations needed to build a half-tunnelled 4.5km motorway link from Mt Roskill to Waterview and to widen the Northwestern Motorway between St Lukes and Te Atatu.

The board expects to take until April to hear the applications, for which the agency has compiled more than 30 volumes of technical reports and intends producing 37 witnesses in the hope of starting five years of construction before Christmas to complete Auckland's 48km western ring route as an alterative to the harbour bridge.

It will also consider 252 submissions, of which about 150 oppose the long-awaited project compared with 30 indications of support, before announcing its decisions in June.

The agency and its predecessors have spent more than 10 years investigating the project, which included choosing the Waterview route instead of one along the Rosebank Peninsula, the preference of the former Auckland Regional Council.

After community opposition to a largely surface motorway, the former Labour Government announced plans for most of the new link to run through tunnels.

But the agency opted for a shorter pair of tunnels along a modified route after National took office and decided the earlier proposal would be too costly while providing only two traffic lanes in each direction, compared with three lanes now being proposed for what it has deemed one of seven "roads of national significance".

That will mean State Highway 20 will be extended above ground for about 1.8km from Maioro St in New Windsor, mainly through a green belt beside Oakley Creek, before entering the tunnels towards the western end of Alan Wood Reserve.

About 205 houses will be demolished in Owairaka and Waterview, and 83 more will have to make way for a wider Northwestern Motorway, for which the causeway through the Waitemata Harbour to the northern end of Rosebank Peninsula will be raised and a new cycle path provided.

Although the project is faced with a long list of challenges, including how to protect Oakley Creek and the Motu Manawa-Pollen Island Marine Reserve, the proposed emissions towers and their associated ventilation buildings have become a focus of particular community concern.

The Transport Agency initially proposed to bury the tunnels' southern ventilation building, but upset the former Auckland City Council and neighbouring residents by changing its mind on cost and technical grounds.

That issue has been picked up by the board of inquiry in a memo issued ahead of today's opening session, outlining what it considers "important matters that NZTA [the agency] should respond to".

The memo says the southern building is acknowledged in evidence to be better if built largely underground, and seeks more information about technical and cost implications.

It also wants a breakdown of costs of filtering vehicle emissions, a proposition which the agency says will be prohibitively expensive and unnecessary as these are dispersed high into the atmosphere.

The Auckland Kindergarten Association is pressing for its Waterview kindergarten, which is just metres from the site of the northern tower, to be relocated permanently rather than just temporarily on the other side of the suburb's neighbouring primary school.

What it is: 4.5km motorway link between State Highways 20 and 16, including 2.5km of twin tunnels, plus widening the Northwestern Motorway between St Lukes and Te Atatu.

Cost: $1.75 billion

Houses to be demolished: About 205 in Waterview and Owairaka and 83 for the Northwestern Motorway widening.

Timetable: National consenting hearing from today for a possible construction start in October. Project likely to take until latter part of 2016.