An Australian man has saved a woman and a six-year-old girl at a popular New Zealand swimming spot and then went home to get his dive gear to recover the body of a young man.

Rob Horne, who grew up in Western Australia and lives in Cromwell, was with a group in a car heading to lunch at a Central Otago vineyard on Saturday afternoon when a woman flagged them down.

"She goes someone's drowning, there's a kid - they're drowning, they're drowning," he said.

He knew the woman who flagged them down because he'd cleaned out her irrigation pond earlier in the day using his dive gear.

The woman was feeding ducks with her grandson when she saw four people in trouble at the Bannockburn inlet, a popular swimming spot.

Mr Horne said he was in the middle seat of the car and jumped out with a friend.

"I just jumped out and ran as fast as I could and chucked my shirt off and my boots and dived straight in."

They saved the woman and the girl who were a few metres out in water.

"When I grabbed the lady she had just gone under the water. She was going."

The girl was on the woman's shoulders.

Then a man yelled "my son, my son".

"I thought what's going on? Is there another person? I can't even see him."

Mr Horne and his friend swam out to the pontoon but the visibility in the water was poor - only about a metre - and they couldn't find the young man.

He got his dive gear from home, 5km away, and after a 20 minute grid search, found the young man's body.

He believes the group of four had recently moved to the area and they couldn't swim.

He believes the young man had the girl on his shoulder and then got into trouble and passed the child to the woman but there was no sign of the man by the time he arrived.

Mr Horne said he doesn't want to be called a hero because someone had died but felt it was a fatality that didn't need to happen.

"I just think learn how to swim or don't go in the water" he said.