Walking and cycling campaigners are preparing their own 50th birthday bash for the Auckland Harbour Bridge next month - a protest crossing the weekend before the official anniversary.

"It's time to get our marching boots ready," says the Getacross Campaign on a website which has registered about 10,300 supporters.

The campaigners plan to march or pedal north across the bridge from 9.30am on Sunday, May 24.

They say the event will be a protest, rather than a celebration of the bridge's 50 years, given the barrier they believe it represents for Aucklanders wanting to reduce dependency on cars.

"But rather than commiserate half a century of denied access, we invite Aucklanders to attend the demonstration march and show their support for walking and cycling on the bridge," said Andy Smith of Walk Auckland.

Co-organiser Bevan Woodward said the protesters had chosen a day and time to avoid interrupting traffic, and hoped the police would support their safe passage.

The Transport Agency has said it will not allow the protest to go ahead.

Auckland Regional Council leaders met the agency on Friday and asked for a lawful trial crossing.

But the agency said that could only happen if the council stumped up $30,000 in traffic management and related costs.

Although the agency has yet to finalise plans to mark the bridge's anniversary on May 30, it has shied away from organising an official crossing that day, fearing it could cost as much as $1.2 million to close it to traffic for six hours and to cope with up to 320,000 pedestrians.

Waitemata road policing manager Superintendent John Kelly said the police had not yet decided how to respond to the Transport Agency's refusal to allow the protest, wanting first to meet all parties "to try to find a way to make everyone as happy as we can".

A crowd of 106,500 people streamed across the main truss bridge over eight hours of May 25, 1959. Some surviving members who made that walk may have their hopes of a repeat performance dashed.

Agency regional director Wayne McDonald said it had a consistent policy of never authorising protest marches, given the need to guard the safety and efficiency of a key part of the motorway network.

"It's a motorway and we are always interested both in maintaining the safety of road users and any protesters, as well as making sure there is no disruption to the use of the network."

Asked about a decision by police in 2004 to override the former Transit NZ and allow 2000 hikoi marchers over the bridge, after receiving warnings of disorder if they were stopped, he said: "The police are the police - they dealt with the situation in their own way".

Mr McDonald said the agency was telling the regional council that if it wanted to explore a trial crossing, that should be done at the council's cost, just as the Auckland Marathon organisers were charged for using the bridge each November.

Mike Lee, chairman of Auckland Regional Council, said an invitation by agency chairman Brian Roche for his council to pay $30,000 for a one-off trial crossing meant a mass protest seemed inevitable.

"I've tried to head this off by arranging a legal way for cyclists to go across the bridge. But the powers-that-be have taken five months to say there's no legal way unless you spend $30,000 of your ratepayers' money, which we don't now have.

"This is typical of the 'no-can-do' of a New Zealand that talks big about a cycleway from North Cape to Bluff but can't organise its way to have a trial across the harbour bridge."

Mr Lee said his council was in an invidious position, as it was seriously short of cash to pay for public transport projects after the Government ditched a regional fuel tax, and it needed extra subsidies from Mr Roche's organisation to keep the ball rolling.