Acting Prime Minister has revealed infection after a "serious" operation was behind his lengthy absence from Parliament.

The 74-year-old NZ First leader has been secretive about the operation that put him out of service for four weeks since returning last week, repeatedly telling reporters it was a private matter.

Prime Minister Jacinda Ardern in August told media Peters was taking time off after a "small surgical procedure to address an old leg issue" she believed to be related to an old rugby injury.

But questions were raised after Peters ended up away significantly longer than expected.

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While the deputy Prime Minister has previously rejected reports he had a knee operation, he on Tuesday told the Newshub's AM Show that complications had kept him away from the Beehive.

"I went into hospital for a serious operation," he said.

"I got an infection, that's what set me back so far."

The Foreign Minister described himself as being back to fighting fit.

Asked further about it on Tuesday, Peters again said it was a matter of privacy.

"It's none of your business. How about I ask you about your life?" he told reporters.

About 2.3 per cent of New Zealand patients are recorded as developing infections after surgery, according to research by the Health Quality & Safety Commission.

"They can make it harder to recover from an operation and increase the time spent in hospital. In some cases they may cause serious illness, long-term disabilities, and can result in loss of life. Infections can also cause financial and emotional stress," a Ministry of Health spokeswoman said.

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Peters is acting as Prime Minister while Ardern is in New York this week.

While "housebound", he lashed out at deputy Opposition leader Paula Bennett on Twitter after she sent him flowers.

He described the gesture as being in "seriously bad taste" and complained the flowers Bennett photographed herself with were significantly nicer than the ones he was actually sent.

"They looked like she just picked them from a local park! Gave them away to an elderly patient. Not knowing their origin, she really appreciated them," he posted last month.

Bennett's rather tongue-in-cheek gesture followed reports on Newstalk ZB that Peters' legal team had discussed Bennett's resignation as a condition for settling legal action against her and fellow National MP Anne Tolley for the alleged leak of details of his superannuation over-payments in 2017.

Bennett had the flowers delivered to Peters in Auckland by a florist.