He confirmed De Vries had speared a kingfish and almost surfaced before disappearing from view.
Mullins said it was probably a "standard blackout" when someone passes out from lack of oxygen.
Mullins and another free diver were alerted by people who were diving with De Vries.
Mullins and a fellow diver searched for about 40 minutes before they found him about 27m down.
"We found him on the bottom and brought him back up."
He said people sometimes died while free diving. One man died in Kapiti a few years ago and another in Tauranga last year.
He said everyone knew the risks, but they didn't expect things to go wrong.
Mullins said where De Vries died was a popular dive spot.
Spearfishing New Zealand chairman Bob Rosemergy said it was a dangerous sport.
"Spearing kingfish has an element of danger to it. It can result in running out of oxygen."
