Between 100 and 150 litres of sewage has leaked on to public land behind a commercial portaloo facility in Avondale.
A witness at the scene said it looked like there has been a "poo tsunami".
The man said sewage had spilt on to an area of grass between the back of a portaloo business and the northwestern cycleway.
"There's s**t on the grass stretching about 100m from the back of a business where a bunch of port-a-loos are stacked," he said.
Blue chemicals had been spread over the sewage - which luckily the man said was masking the smell.
"It's stinky, but it's mostly the chemical smell and luckily it's quite windy, too," he said.
He said there were lots of cyclists and joggers passing by "looking a bit put off."
The fire service was called to the scene, at Patiki Rd, at 10.30am.
The spillage is adjacent to Prestige Loos - a port-a-loo business which carries the slogan "No 1 in the No 2's Business."
The company could not be reached for comment.
A fire service spokeswoman said the spill has been contained, and staff from a nearby business have been tasked with the cleanup.
Auckland Council has been notified of the spill, the spokeswoman said.
Detectives flew to Victoria, NSW and Queensland as part of the investigation.