Corrections have launched an investigation into claims a guard assaulted an inmate at one of New Zealand's toughest prisons.

Labour Corrections spokesman Kelvin Davis was told a serious incident happened at Unit 14 of Spring Hill jail in north Waikato on the morning of Friday, January 27.

He said if the allegations of an unprovoked assault in the prisoner's cell were true, then
Corrections needs to "discipline the person in question".

However Mr Davis told Newstalk ZB the person needs to be considered "innocent until proven guilty".

Advertisement

He has asked for CCTV footage from the Te Kauwhata jail to corroborate or refute the allegations but he said he was told CCTV footage is kept for 14 days, which could be problematic.

"It's only long enough if the process works properly, and it doesn't always."

Mr Davis said transparency was important.

Corrections said it has responded to the complainant and started an investigation.

Acting prison director Christine Faull told Fairfax that Corrections learned of the allegations on Wednesday.

"We encourage any prisoner with concerns to raise them with us so that they are investigated appropriately," Faull said.

"We demand a high standard of conduct and integrity from all employees, and as such, staff are expected to role model positive law-abiding behaviour."

She said the vast majority of staff "fulfil their duties with integrity and commitment in what is often a pressured and challenging environment."