An Auckland man, who lives near the spot where a quad bike carrying four people slipped down a muddy bank yesterday, says the road should have been fixed a long time ago.

A woman, aged in her 40s, suffered serious injuries and was winched out after the 50m fall before being flown to Auckland City Hospital.

Two children had moderate injuries and were taken by ambulance to Starship Hospital, while a third child had minor injuries.

Rodney District resident Hugh McKergow said over the past year, heavy rains had narrowed Ashton Road at Whangateau by a few metres in some places.

"All the cars are parked down ... at the bottom [of the road] where it starts to get steep, for quite a long time now," McKergow said.

"People have been either walking or driving in quad bikes or similar things [to go] up the hill."

Six houses at the top of the road were inaccessible by car, he said.

McKergow hoped the crash would encourage authorities to prioritise a fix.

"The work that should have been ages ago just hasn't been done. I hope this will trigger something to happen pretty quickly. They are working on it but it's not passable by ordinary vehicles."

Residents were deeply shocked by what happened, he said.