Labour has pledged a commuter-rail service between Rolleston to Addington, should it win September's election.

Labour is promising to invest $100 million in Christchurch's public transport, including adding commuter rail between Rolleston and the city.

Should Labour win September's election, the money would support bus services in easing congestion and create opportunities for residential and commercial development, the party says.

Labour leader Andrew Little said there were huge opportunities for greater Christchurch as the rebuild progressed, but congestion and a lack of good public transport was holding the city back.

CAMERON BURNELL/FAIRFAX NZ Labour Party leader Andrew Little says a 21st century city needed a "integrated, multi-modal public transport at its heart".

"A 21st century city simply has to have integrated, multi-modal public transport at its heart."

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He said while the National Government did a lot of work on roads, it lacked a "balanced transport policy".

"They've never been big fans of public transport."

Little said the Rolleston to Christchurch line would be a permanent service, with station placement to be determined after council consultation. Timing of the line would be negotiated with local authorities, but Labour wanted to see it up and running "within a few years".

"Rolleston's population has more than tripled in the last decade and is projected to continue to grow rapidly. It makes sense to restart commuter rail with this line. The network will be expanded over time as suitable," he said.

The train line would take commuters to Christchurch Railway Station in Addington, with buses to the CBD. It would use the existing rail corridor with new and renovated stations and double-tracking where needed.

"We know that traffic volumes are growing on the Southern Motorway, we know that public transport is a growing issue for Christchurch and we know that there is an opportunity to do something with existing rail tracks."

He said he had communicated with city personal trainer Tane Apanui, who earlier this year proposed a low-cost commuter rail service for greater Christchurch.

Apanui said he was glad Labour was taking action, but the plan seemed "a little incomplete".

He said the policy seemed backwards because it was the northern arterial route that was congested: "I'm not sure why they're targeting Rolleston first."

For the rail network to function properly, there needed to be trains running from the north and west, he said.

"If you're going to chuck a hundred million dollars at something, you may as well do the lot."

Labour's Canterbury spokeswoman Megan Woods said the party would work with the Greater Christchurch Public Transport Joint Committee and local councils to decide how to spend the $100m.

Not all of it would go towards rail: "The whole point is that we're being non-prescriptive with the money."

National had "completely missed the opportunity" to include modern public transport in the city rebuild, she said.

Little said the money would come from the National Land Transport Fund and there was room to expand the commuter rail in future.

"The $100m commitment assumes we can use existing rail. We will at some point have to double-track some areas and eventually the idea is that the rail that goes as far as Addington, we'll be able to take it into the CBD."

He said it "might" make sense to integrate commuter rail into the CBD Accessible City transport plan, which was under construction, but the immediate opportunity was to make use of the existing rail track.

Christchurch Mayor Lianne Dalziel said she was aware there were "major disruptions in terms of technology" coming, such as electric and autonomous vehicles.

Dalziel said she wanted to be thinking in terms of transport corridors rather than modes of transport.

Labour had promised a significant sum of money and she wondered if rail was the best way to achieve the goal.

Rail would be considered in the review of the public transport plan, she said.

Commuter rail has been a hot topic in Christchurch.

Labour made a similar promise in 2014, when former leader David Cunliffe announced $100m for a "modern rail plan that will unclog Christchurch's roads".

Apanui's proposal sought crowd-funding to get off the ground in March. The service, called Dash Rail, did not get close to its $1.8m target.

The proposal disagreed with a 2014 Environment Canterbury report, which found rail was not viable because of the expense.

Apanui took the proposal to the Greater Christchurch Public Transport Joint Committee, where committee members voted not to proceed with his plan.

Apanui said he would be open to working with Labour on the project.