National is promising to fully fund an extension to the Christchurch Northern Motorway at the cost of $800 million in next year's budget, if it leads the next government.

National's transport spokesperson Chris Bishop said funding it in the next government's first Budget means construction can begin next year.

The party said it would fully fund the Belfast to Pegasus Motorway and Woodend Bypass, "the next major transport project Canterbury needs", said Bishop as he made the announcement in Christchurch.

"Canterbury is the engine room of the South Island... North Canterbury is one of the places where that growth is being felt most strongly."

Bishop said the Belfast to Pegasus Motorway and Woodend Bypass would extend the Christchurch Northern Motorway around 11 kilometres north, from just south of the Waimakariri River to beyond Pegasus and Ravenswood.

"SH1 approaching Woodend already carries around 21,500 vehicles a day, around nine percent of them freight, and that is expected to increase to around 28,000 vehicles a day by 2048."

He said that pressure was being felt in Woodend itself and at peak times it can take up to 20 minutes to travel through the town, while heavy through-traffic makes it harder for locals to access shops, schools and other parts of the community.

"There is also a significant safety problem. Between 2014 and 2023 there were 280 crashes on SH1 through Woodend, including three fatalities and 25 serious injuries.

"The new motorway and bypass will fundamentally change how this corridor works."

He said the project included widening the southern section of SH1 from two to four lanes, a new four-lane motorway bypass to the north, major intersection upgrades and new bridges.

NZTA estimated the project will save motorists up to 10 minutes at peak times, reduce traffic through Woodend from around 21,500 vehicles a day to 8000, and reduce deaths and serious injuries by 78 percent, Bishop added.

It would also make the corridor more reliable for people and freight travelling to and from Christchurch, Christchurch Airport and Lyttelton Port, he said.

It wasn't a project "starting from scratch".

"Because of National's work in government, the project is ready to go and is the next investment-ready project on the Major Transport Projects Pipeline."