The Destiny Church plans to defy authorities and march 20,000 people across the Auckland Harbour Bridge to protest against civil unions.
The Auckland motorway police say it would be a "nightmare" for the motorway network and have supported a decision by Transit New Zealand to decline an application by Brian
Tamaki's Destiny Church.
Pastor Andrew Stock said the church had given three months' notice of the event planned for Saturday, March 5.
"We have done everything within our power to do things right by the authorities. The issues we are marching for are close to the heart of many New Zealanders who uphold traditional family values and respect the institution of marriage. We did not launch this event lightly and have no intention of cancelling it," said Pastor Stock.
Inspector Dave Walker said the march would be a "nightmare" for the Auckland motorway network, with certain gridlock. There were massive logistical problems with assembling such a large group and the impact on the rest of the community and road users had to be considered.
About 160,000 vehicles crossed the bridge on a Saturday.
Mr Walker said other events, including a fun run and walk last year involving up to 5000 people, were declined for safety reasons.
There were potential structural impacts on the bridge as it was not designed for pedestrian use, he said.
"Also, a march such as this attracts people who just want to walk on the bridge, so it is difficult to control numbers, which then becomes a safety and structural issue."
Mr Walker said police facilitated last year's hikoi against the foreshore and seabed legislation to keep the peace and avoid an "invasion" of the carriageway. That was done after discussions with Transit engineering staff and other experts.
"Police would not expect [that] any group, including Destiny, would encourage its followers to put their lives and the lives of others at risk through unauthorised walking on the bridge."
Mr Walker said the police were concerned the organisers did not appear to have a back-up plan and had encouraged them to look at options that did not involve the bridge.
Last August, followers of the Destiny Church staged an "enough is enough" march in Auckland against the Civil Union Bill.
