The government is investigating whether to place a toll on the Transmission Gully motorway - which is set to open in 2020.

Photo: RNZ / Rebekah Parsons-King

The 27km highway will stretch between Kāpiti and Northern Wellington, and is expected to cost $850 million.

Transmission Gully is New Zealand's first Private Public Partnership for a state highway, which means it is being financed, designed, constructed and maintained by the private sector on behalf of the public sector.

Transport Minister Phil Twyford said the partnership - brokered under the National government - caused the cost of the project to drastically increase, and tolling could be a way to offset that cost.

However, that idea was rubbished by the Automobile Association and local councils.

AA spokesperson Mike Noon said the proposed toll was ridiculous.

"The AA's policy is, look, we support the possible use of tolls to get a project you know, over the line, to maybe happen if it wasn't going to happen or bring it forward," Mr Noon said.

"What we have here is a project which is 50 percent completed so it looks retrospective and it looks like revenue raising to be honest ... that toll wouldn't be paying for the road because it's already funded."

Photo: RNZ/Philippa Tolley

The Transport Agency suggested the toll in a February briefing to Mr Twyford.

The agency warned that without a toll, there could be an increase in demand for road travel and a small reduction in the use of public transport.

The tolling would help counter losses incurred from commuters moving back to their cars and might also manage demand for the road, the agency said.

Essentially some road users who did not want to pay a fee to travel on Transmission Gully would have to take other routes, reducing the flow of traffic on the motorway.

Porirua mayor Mike Tana said that was concerning.

"The impact is if that major road is tolled, that means that people who come through Porirua may make the decision to come back through what will be revocated roads, which are currently the highways going through Porirua which will again mean that traffic that should be going on the motorway may make the decision to go through Porirua," Mr Tana said.

He said those motorists were unlikely to stop in the small town, but they would clog the main roads.

The investigation has begun, but no final decisions have been made.

Mr Twyford wanted more information from the Transport Agency, but said he would place a toll on the road if he felt it was the best decision.