Budget documents released under New Zealand's proactive disclosure regime on Thursday highlighted Treasury's fears over the scheme, even suggesting it would come at the cost of efforts to minimise child poverty.

On August 1, hundreds of thousands of low-income Kiwis received their first share of the $NZ350 ($A317) cost of living payment, designed to buffer New Zealanders against inflation.

However, not only did payments miss many who were eligible for the scheme, they also lobbed in the bank accounts of overseas backpackers and lawyers, as well as the deceased.

The cost of living payment - budgeted at $NZ814 million ($A736 million) - was one of the most spruiked measures of Finance Minister Grant Robertson's fifth budget on May 19.

The earliest budget paper to mention the payment was March 25, showing the quick turnaround between the policy's conception, announcement and delivery.

Mr Robertson did take Treasury advice to make the payment temporary and targeted amid warnings it would be inflationary.

The botched payment plays politically into the hands of the opposition, who under new leader Chris Luxon have derided the government as "addicted to spending" and wasteful.

"The government was warned not to go on a big spending spree at this budget but decided to do so anyway," opposition finance spokeswoman Nicola Willis said.

"It's been a debacle from start to finish (but) Grant Robertson pressed play anyway."

The Labour party imagined the payment would be popular, with party chiefs sending out fundraising emails after the cash was sent.

Like Australia, New Zealand opted to cut fuel excise in March by 25 cents a litre, foregoing government revenue to give back to household budgets as inflation bit.

In recent weeks, the cost of fuel has cratered by more than 50 cents, topping out at $NZ3.34 a litre in June, but dipping back to $NZ2.79 on Thursday, according to price aggregator Gasly.

Unlike Australia, which is reinstating the full excise levy next month, Mr Robertson has lengthened it to January, and will not be budging.

"It was important to provide New Zealanders with certainty about that. Also we do know that the price of oil is very volatile," he said.

Ms Willis said the opposition was open to maintaining the cut.

"New Zealanders welcome the fact that there are lower prices at the pump and the fuel excise cut is part of that," she said.

"We have been keen to observe how that progresses ... to this point we have supported that staying in place."

Australian Associated Press