Steph Weisse, Will Teagle and Jordan Short were found floating on surfboards off Sumatra on Tuesday, along with two Indonesian crew, after the powerboat they were on capsized during a storm on Sunday night.

Ms Weisse's partner Elliot Foote, who had paddled off in search of help, was found a short time later on a nearby island.

A third Indonesian crew member is still missing.

The group of Australians and eight others had been in the Banyak Islands off Sumatra to celebrate Mr Foote's 30th birthday.

"He's all covered in rashes and cuts and stuff, all down the side of his body, but he's all good," Elliot's father Peter Foote told ABC Radio on Tuesday.

"They're going to stay on for another 10 days and have a good time.

"I don't know how he can paddle out into the water again, but he will."

Peter Foote had earlier endured mixed emotions when he learned three of the Australians were safe but his son remained missing.

The Sydney father was speaking to reporters in his Bronte home about midday when he received a brief text: "Hey Dad, Elliot here. I'm alive, safe now, love you. Chat later."

Their later conversation revealed an "absolutely terrifying" night at sea, sparked by the boat hitting a large wave and flipping.

The seven on board grabbed what they could float on and tried to cling together before Mr Foote decided to paddle off, his father said.

Rain, the sea current, hallucinations and a new moon played havoc with his mind and body, taking him kilometres off course.

But the 30-year-old put his survival down to breathing, fasting and meditation techniques, Peter Foote said.

"It's had a big effect on him ... but at the same time, he's happy," he said.

The missing boat sparked a major search in the Banyak Islands, led by Indonesian and Australian authorities.

But it was an Australian expat searching aboard his catamaran who came across Ms Weisse, Mr Teagle, Mr Short and two Indonesian crew on Tuesday morning.

Footage from the catamaran captures the rescuers' cheer as they happen upon the floating five.

But questions of "Where's Elliot?" from the boat and "Have you got Elliot?" from the sea quickly dampened the mood.

Mr Foote was soon rescued, but the fate of a third Indonesian crew member remains unclear.

Foreign Affairs Minister Penny Wong expressed gratitude to the Indonesian authorities who led search efforts and Australian officials who assisted.

"We'll continue to provide support to the four Australians and their families," she posted to X, formerly Twitter.

"The search continues for a crew member who is still missing.

"Our thoughts are with them and their loved ones."

Prime Minister Anthony Albanese said he had contacted Indonesian President Joko Widodo to express thanks for the search and rescue efforts.

The Banyak Islands are known as a tourist destination, with white sand beaches and good waves for surfing.

Mr Foote's party of 12 Australians and five Indonesians had been travelling on two boats from Nias to Pinang Island, which is marketed as an island retreat perfect for eco-conscious surfers.

Ten people in one boat sought shelter on Sarang Alu island while the others continued the trip, Nias Search and Rescue Agency said in a statement on Monday.

The resort on Pinang Island later reported to the agency that the boat with 10 passengers had safely arrived, but the other boat had not been seen.

Indonesia is an archipelago with more than 17,000 islands, and ferries and boats are a common form of transport.

With lax safety standards and problems with overcrowding, accidents frequently occur.

Australian Associated Press