Updated 4.50PM: Len Brown's being described as a very successful mayor who was tripped up by an affair.

The Auckland Mayor has decided against having a third go at the mayoralty.

Massey University's Andy Asquith said the romantic tryst with council advisory board member Bevan Chuang was the core reason behind Brown's decision not to run again.

Asquith said if the affair hadn't happened, Phil Goff wouldn't be circling around, likely to run for the mayoralty.

"He was never going to go for a third term, not after this faux pas, and when it became clear that Phil Goff was going to be a candidate, there's no way on earth that the centre-left wants to split the vote and let the centre-right through."

However, he said Brown shouldn't be judged on that one mistake.

He described Brown as having been a very successful mayor, who made fantastic progress with things like the rail loop.

The academic said, when Brown was elected, Auckland needed a single, clear leadership.

"To provide vision for the city and the region, which he gave us, and he's got those crucial infrastructure projects off the drawing board and into the planning stage."

Brown said it's been his absolute honour to serve as the first mayor of a united Auckland.

He was inaugurated in 2010, and won a second term three years later.

Brown said he's discussed the future with his wife Shan and their daughters, and has decided enough is enough.

He said opponents wrote him off from day one, but the achievements of the first five years of the new Auckland have been extraordinary.

Meanwhile, the man who might be Auckland's next mayor is tipping his cap to Brown.

MP Phil Goff has already booked a venue to make an announcement, although he's refused to confirm he'll be standing.

Goff said he's not entirely surprised Brown's going, and he thinks it could have been tough for him to run again.

"But I think this is the time simply to make a respectful comment about the effort that he put in getting the Supercity set-up, that was no small task, and for the absolute enthusiasm that he has shown for things like the city rail link."