A rescue operation was mounted and a helicopter was on its way, but Mr Wilson rescued the woman before it was needed, Snr Constable Griffiths said.
"There was a possibility we were looking at fatalities. The woman herself said she thought she was dead."
Mr Wilson, who has a crib at the mouth, said he was visiting a friend when someone came to the door saying there were people drowning in the ocean.
They rushed down to the beach to see a young man come out of the water and a woman and man about 200m out to sea in trouble.
Mr Wilson borrowed flippers and a buoyancy vest and swam out to the pair, thankful that they stayed together.
"The guy seemed OK to get in under his own steam ... so I grabbed the girl and swam her back and he followed us in."
They caught a wave, which helped them in. Otherwise, he might have been out there a while helping keep them afloat while a rescue was mounted, he said.
Mr Wilson described himself as a strong swimmer who did a lot of diving, but he did not recommend the mouth as a safe place to swim as even on calm days undercurrents could be very strong.
"Without the flippers and flotation, I wouldn't have got the girl."
Snr Const Griffiths said the woman and the 17-year-old were taken to Dunedin Hospital for treatment as they had taken in a lot of water.
A hospital spokeswoman said yesterday they were treated in the emergency department and discharged on Saturday.