If he wins, a byelection in Rongotai will be triggered.
Whanau is an independent candidate but has support from the Green Party. She told Mills previously she wants "low traffic neighbourhoods" so kids can play safely, cycleways can be better used, and people can make the most of their communities.
Incumbent mayor Foster is hoping for re-election, telling Mills in July he thought his fellow councillors were working well together after reports of infighting plagued the council earlier.
"We've done a lot for the city and I'd really like to take that forward."