Organisers of a protest crossing by pedestrians and cyclists of the Auckland Harbour Bridge to mark its 50th anniversary this month, have assured police they will not force their way over.

Getacross campaign spokesman Bevan Woodward said yesterday he was confident thousands of supporters would be at a rally at the southern end of the bridge on Sunday, May 24, with the aim of gaining authorised access.

The campaign's website last night listed 10,410 supporters of permanent walking and cycling paths on each side of the bridge - costing up to $43 million - which the Transport Agency has decided against building.

But Mr Woodward said that if the police blocked a protest crossing, as they say they will do in response to a request by the agency, the organisers would not incite any unlawful or dangerous activities.

"We said very clearly we want to go across but we won't force our way on to the bridge - we are not going to put people's lives in danger," he told the Herald.

Mr Woodward acknowledged the protest organisers were unable to guarantee there would be no breakaway crossings, as happened when 42 cyclists pedalled their way over the bridge from North Shore City to a Getacross rally attended by about 400 people at Pt Erin in September.

"It's simply about Aucklanders of all ages getting out to show their support - it's not about having a few people sneaking over the bridge just to say they've done it."

Waitemata road police chief Superintendent John Kelly said officers intended to enforce the Transport Agency's request to block access to the bridge, and the protesters should consider their own safety ahead of the risk of prosecution, starting with $250 fines for unlawful use of a motorway.

Mr Woodward said: "If 300,000 people [the agency's estimate of the potential turnout] got to walk across the bridge it would raise expectations, and leave them wondering why they can't do that every day.

"It's a real shame for the region - this is one of our most iconic structures and we are not celebrating it."

Agency chief executive Geoff Dangerfield has denied any such fear, saying the main reasons for not organising a public walk were the difficulties of managing such a large crowd and a likelihood of widespread traffic disruption throughout the day.

The agency's board initially considered celebrating the bridge's birthday in style, with a budget of up to $500,000.

But northern regional director Wayne McDonald said last night the budget had been pared back to $30,000 in recognition of tight economic times.

Commemorations would have a strong educational focus, including the distribution of 1000 copies of books about the bridge to schools and school competitions with guided walks beneath the bridge and over its arches as prizes.