Transport Agency designers should have "no problem" relocating Auckland's historic Birdcage Tavern above a road tunnel through Victoria Park, says a leading civil engineer.

North Shore consultant Peter Riley said yesterday that the roof slab of the tunnel's southern portal could be extended and tapered to avoid increasing the gradient for traffic descending from Spaghetti Junction.

The slab could be made thinner but strong enough with techniques such as post-stressing the concrete, without steepening the gradient or reducing a required 5.4m clearance for northbound trucks on the motorway.

Heritage architect Dave Pearson is also supporting a campaign by urban design specialist Richard Reid for the Birdcage to be returned to its existing position overlooking Victoria Park, once the slab is placed over a 440m trench, the centrepiece of a $430 million motorway improvement project.

The Transport Agency, which is required to restore the 123-year-old brick building under its resource consent, has allocated $8 million to $10 million to strengthen and move it to a new position 30m up Franklin Rd.

But although Mr Reid believes the hotel deserves more prominence than being tucked behind the tunnel portal, the agency says returning it to its existing position would increase risks both to the building and to traffic facing a steeper descent.

Northern highways manager Tommy Parker said after the agency announced the choice of a Fletcher-led consortium for a $340 million construction contract this week that a steeper descent would increase the danger to traffic if a fire should break out in the tunnel.

But Mr Riley, a rock mechanics expert and dam-builder who successfully lobbied the former Transit NZ to dig twin tunnels rather make an environmentally destructive cut through Johnstones Hill on the Northern Gateway motorway, said tapering the Victoria Park portal was the simpler and more sensible option.

Although not purporting to be a structural engineer, he said he had managed enough major projects "to know it can be done".

"It's an engineering problem and it can be solved," he said.

Mr Pearson, who measured the Birdcage for engineers preparing resource consents for its move, was confident it could be replaced with little of the extra risk feared by the Transport Agency.

"The most difficult bit would be the initial move, because of the enabling work."

But he believed it would be easier to roll the Birdcage back downhill once the hard work of hauling it up Franklin Rd was over.