Manukau Mayor Len Brown has come under fire for spending thousands of dollars of ratepayer cash to take his wife on a sister-city trip to Japan.

Supermayor candidate Brown was also criticised for opting to fly premium economy on the junket while other members of the delegation flew economy.

Flights for the mayor and his wife Shan cost $7900, and a further $8700 was spent flying three council staff to Utsunomiya, 120km north of Tokyo.

Council chief executive Leigh Auton also brought his wife, Jenny, on the trip but paid for her costs personally.

Brown, who arrives back in Auckland today, said he understood travel was a sensitive issue but added it was crucial to improve relationships with trading partners.

His wife's travel was "appropriate given the nature of the relationship," and because they were hosted by the mayor and mayoress of the city.

"We have to manage them with some maturity, not in the midst of a typically reactive debate.

"There's a lot of mana on the line, and the Asian community are huge trading partners," he said.

"So much of how we do in business relates to how well we relate on a one-to-one personal front."

Managing those relationships would also be crucial to the new Auckland supercity, he said. The mayor was allowed to fly business class under council regulations, but Brown chose not to.

The 28-year bond with Utsunomiya had seen 10,000 people take part in cultural exchanges between the two cities, he said.

Councillor Jami-Lee Ross it was "odd" that council chief executive Auton had opted to pay for his wife, while the mayor hadn't.

"I would have thought if it's good enough for the staff to sit in economy, the mayor could have travelled economy too."

Manukau councillor Dick Quax said he didn't see the value of the trip to ratepayers.

He said the association with Utsunomiya had little benefit to Manukau.

He said: "It's council policy that the mayor can be accompanied by the mayoress, and they've taken full advantage of that policy."

Both Quax and Ross voted to approve the trip in September, though Quax said he "expressed reservations".

In August, Brown led a trade delegation, without his wife, to Samoa, American Samoa and Tonga at a cost of $31,000.

* $7900: Cost for Len Brown and wife Shan to fly.
* $8700: Cost for three staff on same trip.
* $31k: Brown's delegation to Samoa, American Samoa and Tonga.