Gridlock is predicted to worsen across the Wellington region after Transmission Gully and the Kapiti Expressway are built.

Hardest hit will be Wellington city, as people from Porirua and the Kapiti Coast ditch public transport in favour of a faster, cheaper journey into the capital on the new four-lane highways.

The predictions are contained in a report commissioned by Greater Wellington Regional Council, which warns that local roads could struggle to handle the additional tens of thousands of cars hopping off State Highway 1.

The report, by independent consultant Opus, was completed in December but has not yet reached the regional council table. It will be discussed at committee level this month.

Paul Bruce, the regional council's Green party representative, called the report a "damning indictment" of the Government's $3 billion upgrade of SH1 between Wellington Airport and Levin, known as the Wellington Northern Corridor.

It includes the Transmission Gully highway, Kapiti Expressway, Basin Reserve flyover, and the undergrounding of Buckle St.

"They're throwing away money on these roads when they can't afford to . . . inducing more car use is no good for anybody," Mr Bruce said.

The Opus report forecasts congestion in the capital will dip slightly between now and 2021, with the cost of running a car expected to increase more than public transport fares.

But once most of the northern corridor is in place after 2021, Wellington city will experience a 50 per cent increase in congestion during the morning peak by 2031, and an 80 per cent increase by 2041.

Population growth and more employment opportunities in Wellington are also contributing factors. By contrast, Porirua and Kapiti will have 96 per cent less congestion during the morning peak as Transmission Gully and the Kapiti Expressway soak up the traffic.

Mr Bruce said the Government needed to invest in public transport rather than new roads, otherwise a peak-hour toll would be "inevitable" to discourage cars from central Wellington.

According to the report, an extra 18,643 vehicles will descend on the capital during the morning peak in 2041 - a 26 per cent increase on 2011. That suggested the roading upgrades ran counter to Greater Wellington's aim of encouraging more use of public transport, Mr Bruce said.

Council chairwoman Fran Wilde rejected the claim, saying the report would have told a different story if it had taken into account several public transport projects still being investigated.

They include the proposed new public transport network between the central business district and southern suburbs, as well as the introduction of smartcard ticketing for buses, trains and ferries.

The Opus report was one of several "ongoing tests" because "no model can 100 per cent accurately predict the future," she said.

NZ Transport Agency central region director Jenny Chetwynd, said the agency's "more detailed" analysis showed the SH1 upgrade would deliver a quantum improvement in safety and efficiency.

Automobile Association spokesman Mike Noon said he did not think motorists had anything to fear from the new highways.