The contractors proposed the first alternative after choosing technology capable of boring under Great North Rd without need for a "cut and cover" section at the Waterview end of the 2.5km tunnels, as originally planned.
They have ordered a 100m-by-15m boring machine from an overseas supplier for assembly by April next year, at a cost of $54 million to $56 million, with a right to sell it back after the project is completed in 2016.
The Transport Agency said before Christmas that the site should be investigated to avoid cutting a disruptive trench through the busy arterial road to pipe fumes from the tunnels to a venting tower above Oakley Creek.
But a community liaison group for the project was backed by the Albert-Eden Local Board and the council committee in opposing that, saying the tower would be closer to the school and more of an eyesore.
Transport Agency northern highways manager Tommy Parker now accepts that the first alternative site was "too close and prominent to the heart of the community".
He told the Herald that although the newly proposed site was still on the school side of Great North Rd, it was about 100m north of the tunnel, close to a multi-level interchange to be built between a new 4.5km motorway link from Mt Roskill and the Northwestern Motorway.
But Mr Parker said the agency did not have a preference between that site and the one approved by the board of inquiry, and the agency would not force the issue.
"If we want to go for the new site, we would have to lodge a change of designation, and obviously we would only do that if we felt there was overwhelming support from the community," he said.
"We genuinely don't have a preference, but when a new opportunity came to light we felt it was appropriate to discuss it with the community."
Although there would be less disruption to Great North Rd traffic in building the tower on the same side as the tunnel portals, some excavation would be needed near the busy route.
That would enable the tunnel-boring machine to make a u-turn after digging a northbound tunnel to go back south to Alan Wood Reserve in Owairaka.
A 100m trench will also be dug for the machine in the reserve, from where a 2km section of new surface motorway from Mt Roskill will ultimately run through the tunnels to Waterview.
Mr Parker acknowledged that the northern tower would still be "quite prominent" for traffic heading towards Waterview.
Waterview resident Bill McKay, spokesman for the Northwest Community Association and Living Communities, said that the new proposal was unlikely to win support as it would be far more prominent than the board of inquiry approved.
VENTING
* Proposal 1 15m tunnel emission tower on the opposite side of Great North Rd from Waterview Primary School - approved by Government-appointed board of inquiry.
* Proposal 2 Tower between tunnels' two northern portals, on Great North Rd side of the school - dropped after community opposition.
* Proposal 3 Tower on Great North Rd side of the school, but 100m north of the portals - up for public consultation.
