The female instructor stopped the class and three other gym staff members came over to help, she said.
Another member of the class rang for an ambulance, while one of the staff members stayed with the man.
In the meantime, the instructor resumed the class, the woman said.
The class continued for about 25 minutes until the ambulance arrived and he was taken to hospital.
"I felt really bad for him. They totally should have finished the class," she said.
"It was a poor decision. Les Mills really handled this badly."
A spokesman for St John Ambulance confirmed that the man was taken to Auckland City Hospital.
Les Mills spokesman Guy Needham said the class should have been stopped completely.
"The health and safety process wasn't followed," Mr Needham said.
"We're obviously investigating with the people concerned and the appropriate action will be taken," he said.
The man was yesterday in a serious condition in hospital, he said.