TRAIN SET: Labour leader Phil Goff is backing Auckland Mayor Len Brown's plan for an inner city underground rail loop.

If Labour was elected to government it would pay $1.2 billion or half of the cost of the proposed inner Auckland rail loop, Labour leader Phil Goff told around 100 supporters at an open air party meeting today.

The loop is projected to cost $2.4 billion.

Auckland City would have to pay half, and a Goff lead government would pay the other half by cancelling a "holiday highway" north of Auckland.

Goff said to become the world class city Auckland wants to become, it needs a world class transport system.

The current western and southern motorways were "bloody nightmares" he said.

Labour wanted to support Auckland Mayor Len Brown's plan for an inner city underground rail loop which would double the amount of trains the Britomart transport system could handle.

He attacked Transport Minister Steven Joyce, saying he was "mesmerised" by motorways and was planning to build a $1.7 billion motorway from Puhoi to Wellsford, which would link popluar beach destinations.

"We wont be going ahead with the holiday highway out of Auckland," he said.

Goff said it was fiscally irresponsible to consider building it now.

Labour would use $320 million saved from not building the highway to improve State Highway One, particularly at blackspots.

Labour's transport spokesman Shane Jones said some of National's new roads were necessary but they were closing off other transport options.

"National crossed the centre line three years ago, and has been driving down the wrong path ever since," Jones said.