Watercare says the sewage spill at Auckland's Civic Theatre was caused by a build-up of cooking fat blocking and overflowing the wastewater pipe.
A spokeswoman said their crew found the pipe blocked with fat, but was cleared out by 5:30pm yesterday.
"The only connection upstream of the Civic Theatre is the IMAX complex which has a large food hall. This suggests that one or more food outlets were not maintaining their grease traps, allowing fat to enter the public wastewater network."
Watercare will today be sending out a team to speak to fast food vendors, to educate them on the correct disposal of cooking fat.
"We're going to send a team round there today with essentially pamphlets and posters showing them the correct disposal of fat, oil and grease," she said.
"There is actually nothing wrong with our pipe, but when people pour fat down the drains, it blocks our pipes as it cools down."
The sewage spill forced tonight's scheduled concert The Last Waltz to be moved from the Civic Theatre to the Aotea Centre.
No damage was caused to the main theatre or stage area, however "grey water" had flown into backstage dressing rooms across two levels.
The sewage pipe, which runs external to the Civic Theatre, overflowed after the main drain had a blockage which backed up into two floors behind the Theatre.
Auckland Live director of regional facilities, Robbie Macrae, said the overspill had not affected the main stage, auditorium or foyers.
"It was pretty odiferous - that was the word I would use," Macrae said.
"We've had Envirowaste in giving the two floors a full clean, and totally disinfected.
"As a precaution we have moved tonight's concert The Last Waltz - which was going to be in the Civic - into the Aotea Centre."
The Civic has been given the all-clear to go ahead with the 48 Hour Film Festival scheduled for this weekend.
The problem with the blocked pipe has now been fixed and there will be no issues going forward, he said.
All ticket holders to The Last Waltz will need to be reissued tickets to the ASB Theatre, Aotea Centre. This can be done at the Aotea Centre Box Office between 12 noon and 4pm.