Incumbent Wellington Mayor Justin Lester is currently behind in the race and facing a tough battle to win.
Councillor Andy Foster, backed by Sir Peter Jackson, has a narrow lead over Lester, according to the latest progress results.
As of 4:16pm, Foster had received 24,108 votes compared Lester's 23,393 votes. That's a margin of just 715 votes.
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Wellington City electoral officer Warwick Lampp says approximately 90 percent of the votes received have been counted so far.
However a large number of last-minute votes meant a preliminary result won't be available until Sunday.
"Today, we collected 5,563 votes, including special votes," he says.
"This unprecedented volume of last-minute votes means we won't know the preliminary result until midday tomorrow, so this progress result may change."
A final result won't be available until mid-next week.