Police confirmed there had been a sudden death thought to be related to a medical event.
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St John paramedics were called to the scene at 7.04am, a spokesman said, and sent two ambulances and a rapid response vehicle.
Victim a 'valued member'
Stewart said the victim was a valued member of their centre and would be missed by the Manurewa team.
"Our deepest condolences go to his family and loved ones."
Stewart also paid tribute to the staff members at the site this morning, who he said acted quickly during a difficult time.
"These types of incidents are always distressing for staff and members of the public and, as a result, we have closed the centre until further notice to support our people," he said.
A message by the affected pool and leisure centre was posted to its Facebook page telling the public that they would be closed.
"Due to unforeseen circumstances, our centre is closed until further notice. We do apologise."