Since the formation of the "Supercity" Auckland Council in 2010 both mayors Len Brown and Goff have had Labour affiliations.
They ran as independents but with public party endorsement being made once they were campaigning.
In the past week, Labour-aligned councillor Richard Hills said he was dropping his plans to run for the mayoralty.
This leaves Labour's Manukau Ward councillor Efeso Collins as the only declared left-leaning candidate.
There is still to be a confirmed candidate from the centre-right, with restaurateur Leo Molloy and Heart of the City chief executive Viv Beck yet to formally confirm.
Returning candidates Craig Lord and Ted Johnston, along with newcomer Jake Law, are the others who have thrown their names into the hat.