Maritime New Zealand has launched an investigation into a fatal boat accident in Riverton.

Three people died when their boat capsized while crossing the Riverton bar on Saturday afternoon (3 Aug).

Two others were rescued.

Coastguard Riverton skipper Ross McKenzie said the three who died were locals and were known to the Riverton community.

Police Southland area commander Inspector Mike Bowman said a member of the public saw the boat get into distress and quickly alerted police.

The Coastguard were doing a training day at their base and were able to quickly respond, he said.

"We were able to get on site really quickly."

The bar "can be quite tricky" but why the boat upturned was being investigated.

McKenzie said he had a safety message for people.

"Any time you're on on the boat ... wear a life jacket, it's something that's going to keep you at least safe for a while until rescue services can get to you. Know your conditions and look for the weather.

"I can't stress enough the importance of wearing those life jackets."

[The NZ Herald and Stuff have named the deceased as former Southland farmers Richard and Heather King, and retired architect John McCulloch.]