Public transport campaigners are proposing alternatives to a $1.65 billion highway from Puhoi to Wellsford for fractions of the cost.

Labour and the Greens are also pressing the Government to give priority instead to a $1.5 billion rail tunnel through central Auckland to double Britomart's capacity, a project strongly advocated by the main Super City mayoral contenders.

The Campaign for Better Transport says almost half the benefits, such as journey time savings, predicted for a 38km dual-carriageway extension of the $356 million Northern Gateway toll motorway could be achieved for 20 per cent of the cost of what the Government has designated as one of seven roads of national significance.

It has used figures obtained from the NZ Transport Agency under the Official Information Act to compile a report listing improvements to the existing main road to Northland from investments of $160 million and $320 million respectively.

Both options would include a $50 million bypass of Warkworth and safety upgrades such as extensive median barriers for notorious black spots on a road on which 41 people died between 2000 and last year.

"The options we have put forward are far more cost effective, will save lives and can be implemented far sooner than the current Transport Agency proposal," said campaign convenor Cameron Pitches, after the Auckland Regional Council's transport committee endorsed the alternatives last week.