Auckland City Council design advisers have lent weight to a call for Auckland's Birdcage Tavern to be included in a public plaza above a $430 million motorway tunnel through Victoria Park.

Although the Transport Agency intends moving the 19th century building 30m up Franklin Rd, it says returning it to its existing site once the northbound cut-and-cover tunnel is completed by early 2014 would be "prohibitively expensive".

Senior agency official Tommy Parker also said yesterday that shifting the building in the first instance would be risky enough, but moving it a second time would be "highly risky".

But he acknowledged panel of urban design experts had called for the hotel to be returned to its original site, as first advocated by Auckland architect Richard Reid, who is also citing support from Auckland Regional Council's transport committee.

Mr Parker said although the agency held a resource consent to move the Birdcage up Franklin Rd, final design details had yet to be approved by the city council.

That could not happen until a contracting alliance was hired to dig the tunnel and widen the surface motorway along St Mary's Bay to the bridge, after drawing up a final design within parameters set for the agency by its resource consents.

He said the final details would be referred back to the urban design panel, and then the city council itself for ultimate approval of a masterplan.

The design panel's initial recommendations, contained in previously confidential minutes of a meeting late in 2007, say the Birdcage should be returned to its existing site "to become part of an integrated urban proposal" also incorporating such other significant buildings as Victoria Park Market and the now-derelict Logan Campbell Free Kindergarten.

But Mr Parker said the Birdcage, if returned to its site after the tunnel was built, might have to remain under the shadow of the existing Victoria motorway flyover for 30 more years.

The flyover will continue to carry southbound traffic until its ultimate replacement, not yet designed.

He said returning the hotel would require extending the tunnel's southern portal, which would be "prohibitively expensive" because of a resulting need to dig back toward the central motorway junction to avoid the slope becoming too steep for traffic.

But Mr Reid said he and an agency engineer had discussed the possibility of tapering the portal roof instead.