John Banks has admitted he could lose the Super City mayoralty on Saturday unless people in Auckland City and on the North Shore vote for him in big numbers in the dying days of the campaign.

"If they don't turn out and vote, we are going to get a Labour mayor from South Auckland in charge," said the Auckland City Mayor.

Postal voting returns show Mr Banks' strongholds of Auckland City and North Shore are well down on Manukau, where rival Len Brown has huge support.

Union leaders and the churches have mobilised South Aucklanders like never before to get out and vote for Mr Brown, while the Banks camp has struggled to match the effort, particularly on the North Shore, where the vote is barely more than 20 per cent.

With an upsurge in votes expected in the final days, Mr Banks yesterday made a rallying call to his supporters by way of a scathing attack on Mr Brown.

Mr Brown, he claimed, was simply not up to the job and had buckled and collapsed under intense scrutiny over his council credit card. He compared him to former Auckland City Mayor Dick Hubbard - "another good bloke simply not up to it".

"If all the people who want to vote for John Banks vote for John Banks, we will win on Saturday. If we just leave it to the voters of South Auckland, we could lose."

Last night, a spokesman for Mr Brown said the Manukau Mayor was not interested in responding to Mr Banks. "This is desperate and divisive personality politics from a man with no policies for Auckland who is behind in the polls.

"Banks' recent negative comments are why people should vote for someone who can unite this city," the spokesman said.

This was a reference to Mr Banks' South Auckland barbs yesterday and also last week that "we don't want South Auckland replicated across the rest of Auckland. His [Brown's] city is a social disaster".

Prime Minister John Key told one TV network yesterday that Mr Banks, a former National Cabinet minister, could definitely still win the mayoralty.

"But ... whether it's John Banks or Len Brown or anyone else, we'll work with them."

By yesterday, just 27 per cent of registered voters had returned their ballots. Election organiser Dale Ofsoske is predicting a final turnout of between 45 and 50 per cent.

- additional reporting: Isaac Davison and Elizabeth Binning

This weekend's Super City result threatens to be geographically lopsided, as voter turnout in South Auckland easily outnumbers those of North Shore and Waitakere. With the postal deadline tomorrow, voters are being urged to have their say.

South Auckland: 30 per cent-plus.
Auckland City: Mid 20s.
North Shore/Waitakere: Low 20s.