Labour's leader is yet to reveal how much it will cost to halt fuel tax increases for the next three years.

The party's campaigning on keeping the fuel tax at the current rate if elected, while the government yesterday promised no increase next year - and smaller increment increases after that.

Fuel excise taxes were due to be increased by 12 cents from 1 January 2027, then increase annually by 6 cents and 4 cents heading into 2029 - a total increase of 22 cents.

Instead, the tax will go up by 5 cents every six months, beginning in 2028 through the end of 2029 - a total increase of 20 cents.

Fuel tax revenue goes into the National Land Transport Fund which pays for building, maintaining, and upgrading the roading network.

Transport Minister Chris Bishop said the government would top up that fund nearly $1.5 billion to keep up with the required work.

Cabinet agreed to use its $450 million rainy day fund created in this year's Budget to help cover that - and will find the rest elsewhere.

Meanwhile, National said Labour's policy would leave it with a $4.6 billion shortfall.

Labour leader Chris Hipkins disputed that, saying National's figure included costs extending beyond the next Parliamentary term.

But he would not say what the true figure would be, saying the party had "an idea" of what it would cost.

"It will be a lot less than that $4 billion figure that they've put out there," he told Morning Report.

"We will update our own figures and put them out there once the government have opened the books," he said, referring to the pre-election economic and fiscal update.

Hipkins said Labour would aim for as much funding continuity as possible, and prioritise maintenance and public transport over expensive new roads.