Police are investigating how its officers forgot about a man in a Waiararapa lock-up and instead left him alone in his cell from Saturday until Monday morning.

The man had been into the court cell on Saturday morning after appearing in Masterton District Court on a breach of bail charge.

Despite being granted bail by the Justice of the Peace, police and court staff discovered him still in the court cells on Monday morning.

Fairfax reports he had access to water from a drinking fountain but no food throughout the weekend.

Wairarapa Area Commander Donna Howard said doctors immediately checked the man before he was taken home, with an investigation now launched into how the incident occurred.

"Police have apologised to the man and his family for the distress caused," she said.

"The priority is to determine how this happened and to continue to support the man and his family."

"We are working with the Ministry of Justice and police have also referred the matter to the Independent Police Conduct Authority."