Auckland's new rail lines could be ready within six years, but only if Labour is still in power

Auckland commuters could be using two new light rail networks across the city within six years, the Government says.

But that will depend on who is in power. National warned yesterday that the planned airport line and northwestern line could be scrapped if it was elected in 2020.

The party said it remained sceptical about the costs and the need for light rail in Auckland, and its preference was for busways.

That was despite early, unsolicited interest by the New Zealand Super Fund in building and owning and operating the rail lines.

The Labour-led Government surprised yesterday by announcing an immediate start on not one but two light rail networks in Auckland.

An earlier start date for the CBD to airport line had been expected, but the deadline for a northwestern line to Westgate and eventually Kumeu was, until yesterday, uncertain.

Transport Minister Phil Twyford said their completion would depend partly on how long it took to find backers for the projects. But he was hopeful that trains would be running by 2024.

"My expectation, looking at the work we've had done by officials, looking at similar light rail projects around the world, you're probably looking at around six years.

"Possibly a little bit longer if you take into account the procurement and tendering process over the next six to eight months."

The new networks are also contingent on whether Labour is re-elected in 2020.

National's transport spokesman Jami-Lee Ross said his party was not sold on light rail in Auckland. It was not cost-effective and there was not the population to support it, he said.

Asked whether potential investors in the rail projects should be concerned about National's plans, he said: "Light rail down those corridors is not guaranteed past 2020."

National's policy is to protect the CBD to airport route and focus on a busway, with rail possibly to be built in 2040.

The Super Fund's interest did not necessarily make the projects more appealing, Ross said.

"You've got a Government that will plough anything [into] making light rail work. That will result in huge subsidies … and the Super Fund will be interested in a guaranteed revenue stream."

Finance Minister Grant Robertson said the Government would still run a full tender process despite the fund's interest, which he described as unprecedented.

"We want to … ensure New Zealanders get value for their money," Robertson said.

The Government and Auckland Council have already committed $1.8b for light rail in their 10-year plan.

That would cover less than a third of the $6b cost of the two rail projects. The remainder would have to come through investors or other means.

Twyford said one possible option was land value capture – a special tax on property near the rail line which is deemed to have benefited from the new infrastructure.

The final plan is for two-carriage trams capable of carrying 420 people at a time, and running every five to 10 minutes.

They will cut travel times because they run on dedicated tramways which have automatic right of way at intersections and fewer stops than buses.