The Olympic Pools in Auckland is investigating after two men were found unconscious in a swimming pool at the weekend.
The pair had to be resuscitated by staff and a visiting doctor on Saturday evening after they got into difficulty at the Olympic Pools and Fitness Centre in Newmarket.
The two men - believed to be in their 20s and 30s - remain in a critical condition.
Olympic Pools director Peter Rust said the men appeared to be free divers who were practising holding their breath under water.
They were told not to do that by lifeguards, he said.
They were looking at what happened, as well as reviewing the rules for free divers training at the pools, Rust said.