LOST TOO SOON: Tony Tuatoko (second from the right) with workmates.

One of the "most-loved" instructors at a Wellington gym has died suddenly while taking a class in the main studio.

Tony Tuatoko, 40, was a group fitness instructor at Les Mills Extreme, the Terrace, and Hutt City.

It was understood Tuatoko died while teaching a CX Worx class at Les Mills Extreme on Taranaki St around 6.20pm yesterday.

Les Mills Wellington marketing manager Arpita Dutta confirmed Tuatoko died last night, but would not comment further on the circumstances of the tragedy.

The "entire Les Mills family" was mourning the loss of "one of our most-loved group fitness instructors," she said.

"He was loved by all. His Friday classes were always packed, and he was just a cheeky guy."

Dutta said Tuatoko's grieving family had asked the gym not to talk about his death as relatives overseas were still being notified.

Les Mills was respecting their wishes, but wanted to let people know "it was one of our own".

A spokesman for Wellington Free Ambulance said they were called to Les Mills Extreme for a "medical event", but could not give further details due to privacy restrictions.

Tuatoko described himself on the Les Mills website as having been involved with group fitness for the past six years.

"I love to workout and I love to have fun at the same time," he said on his trainer profile.

Les Mills Extreme posted on Facebook around 10pm last night that there would be no group fitness classes in Studio 1 today as they mourned the loss of one of their "beloved" instructors.

Regular attendees of Tuatoko's gym classes expressed their shock at the news on Facebook, with one describing it as "the most tragic news ever".

Others wrote of how Tuatoko motivated and encouraged them, and thanked their instructor for the passion and energy he brought to teaching fitness classes.

A way to remember Tuatoko was being planned by the gym, and details would be announced to members as well as the wider community once it was confirmed.