Transmission Gully would take almost eight years to build - and the project could blow out to more than $170 million over budget - but support for it appears to be growing reports Kerry Williamson in today's Dominion Post.

Figures in a report issued to The Dominion Post by the Transport Agency show the project's 95 percentile estimate - effectively a worst-case scenario cost - could balloon to $1.19 billion, significantly higher than its current $1.02 billion price tag. It is a "one-in-20" scenario that could be sparked by something as simple as delays caused by bad weather.

The project's preferred route, made public last month, has been touted as $275 million cheaper than the previous designation.

Principal project manager Rob Whight said the higher cost estimate stemmed from the risk of tackling such a massive project. He pointed to other estimates that show $200 million could be saved if construction went off without a hitch.

"Risk will exist all the way until the day you finish the job. That's the reality of any project," he said. "... The trick is to get the balance of risk right."

The report confirms the discovery of a previously unknown "splinter" of the Ohariu Fault, and that land acquisitions alone will cost more than $50 million.

Funding remains the biggest hurdle - local governments have till late next year to find $600 million.

Support for the project appears to be growing. Of the more than 2000 submissions already received on the preferred alignment, 90 per cent are supportive.

The report finds $53 million will have to be spent on land acquisition alone - 188 hectares have already been bought.

Earthworks would cost $130 million, bridges $91 million, retaining walls $67 million and ground improvements $64 million. Investigations into the project have cost $9 million.

Traffic studies predict a 15 per cent increase in vehicle movements on the coastal highway between 2016 and 2026, and an almost 40 per cent increase in heavy vehicles within 20 years.

Since September 2004, State Highway 1 between McKays Crossing and Linden has been closed 51 times.

Delay times on the highway are predicted to increase between 2006 and 2026 by 40 per cent in the morning peak, and 80 per cent in the afternoon rush hour.

"The existing coastal highway was not designed to cater as the primary through-route for the volume of freight and other traffic now using the route," the report says.