Walkers and cyclists planning a protest crossing of Auckland Harbour Bridge on Sunday morning will find it defended by a new security fence as well as by police.

The Transport Agency, which has put up the 1.8m fence this week along the Curran St on-ramp, is also concerned that schools have been among recipients of a mass email invitation by the Getacross Campaign to "a public walk/cycle" over the Waitemata.

The agency said yesterday that it had emailed almost 50 schools believed to have received the invitation, to warn them off on safety grounds.

Getacross spokesman Bevan Woodward said the campaigners had no intention of putting anybody's safety at risk, particularly that of children.

"We were never going to storm the bridge, or force our way on to it."

He said the security fence was beside the point, as the campaigners remained confident of gathering enough supporters to persuade the agency and police to allow them to cross the Waitemata safely on foot.

He said the campaigners would make a strong request to cross the bridge, and expected the police to offer them safe passage, as had been enjoyed by Maori rights marches.

Transport Agency northern director Wayne McDonald acknowledged that Mr Woodward had assured him the protest would be peaceful, but said he was concerned the campaigners were raising dangerous expectations that the public would be allowed to walk over the bridge.

"We respect people's right to demonstrate in support of their views and we welcome an open and honest dialogue on the issues relating to walking and cycling on the bridge," he said.

"But we would be failing in our duties and obligations if people exercising that right were allowed entry to a high-speed motorway where their safety would be put at serious risk."

Mr McDonald said closing the bridge to traffic to accommodate Sunday's demonstration would cause widespread congestion on Auckland's motorway network.

Mr Woodward said the campaigners were not asking for the bridge to be closed, just seeking access to its northbound clip-on lanes only.

This Sunday's demonstration would be symbolically significant as it would be exactly 50 years since 106,500 people walked across the bridge just days ahead of its official opening on May 30, 1959.