A New Zealand police officer who was seriously assaulted while in custody in Tonga last Friday has died.

Kali Fungavaka was allegedly bashed on Friday night (local time) in a police cell where he was being held with up to five others after being arrested for alleged drunkenness outside a bar in Nuku'alofa.

He was taken to Vaiola Hospital in a critical condition but died on Thursday evening with his wife and other family members at his bedside.

Constable Fungavaka, who lived in south Auckland and worked at Otahuhu Police Station, is married and has at least four children.

New Zealand Deputy Commissioner Mike Bush described Const Fungavaka as an outstanding officer, while his colleagues said he was quiet, humble and passionate about his job.

"He was always one of those staff members who could be relied upon when the going got tough," Mr Bush said.

"We have conveyed our deepest sympathy to his partner and family and our thoughts are with them."

Kalisitiane Manu, who was in the police cell with Const Fungavaka, has been charged with "bodily harm" and is due to appear in court on Monday.

Two Tongan police officers have also been arrested for allegedly assaulting Const Fungavaka earlier on Friday evening.

Fairfax reports that Const Fungavaka became involved in a brawl with the Tongan police officers in a bar in central Nuku'alofa just before he was arrested.

Const Fungavaka was in Tonga to attend his grandfather's funeral.

He was awarded a New Zealand Bravery Medal in 2006 for saving a burning woman set alight at an Auckland petrol station.