Phil Goff has accused mayoral rival John Tamihere of spreading "fake news".
The present Mayor of Auckland said he welcomed the Advertising Standards Authority's (ASA) decision to uphold a complaint about a radio advert Tamihere's campaign ran for two weeks.
The advert, which aired on NZME's Newstalk ZB, claimed Mayor Phil Goff was "letting Auckland Transport cut the speed on 700kms of roads to just 30km/h".
"No suburb will escape this crazy plan," Tamihere, who voiced the ad, claimed.
The ASA agreed with the complainant this wasn't true, as many of the roads will actually have limits up to 80km/h under the proposal.
"It is disappointing to see candidates engaging in false advertising, but I am pleased the Advertising Standards Authority has upheld the complaint made by a member of the public," Goff said.
"The people of Auckland expect those running for political office to be truthful and act with integrity - not resort to fake news and fake advertising.
"I hope this is a good reminder to Tamihere to clean up his act."
Tamihere's campaign said the advert omitted the finer details of the plan due to time constraints.
"We did not have time to say that in 30 seconds and chose simply to alert people to the proposal because Auckland Transport had done a very unsatisfactory job of advising the public.
"More people heard our advertisement than Auckland Transport reached with their 'consultation' programme."
NZME told the ASA it relied on advertisers to ensure their ads were truthful.
"Notwithstanding that, we are cognizant of the fact that we too have a responsibility as a broadcaster to ensure that the public is not misled."