AN INTERNATIONAL investigation is underway after a Queensland woman disappeared from a yacht in the middle of the night off the coast of Thailand.

Carmel Brookes, 60, was last seen aboard her yacht, the 14m Moondancer 2, as she and her boyfriend, prominent marine biologist Dr Gerry Goeden, sailed from Langkawi to Phuket.

Dr Goeden told police he got up to take over the night watch from Ms Brookes about 1.30am and found her gone.

The former dancer and government worker's Brisbane-based brother, Bill Heang, said his sister had been excited about the trip after planning it for some time.

"I got a call from Air Sea Rescue on the Thursday morning about 6am saying they had picked up a distress beacon for Moondancer 2," he said.

"From that day on we've been trying to find out what happened."

Mr Heang said five days later he was put in contact with Dr Goeden, who was still at sea, through Australian consular officials.

Dr Goeden explained Ms Brookes had been on the night watch as they sailed off the coast of Trang, between Langkawi Island and Phuket.

He said he got up to relieve her at 1.30am, found her gone and set off the distress beacon.

"He said two bean bags that had been in the life raft were missing, plus the boat hook, so he believed she had fallen in trying to retrieve them," Mr Heang said.

"They were red bean bags and they float but they haven't been found.

Several days later, Australian consular officials called Mr Heang to say they could do nothing more to help because no official report had been made to Thai police about his sister's disappearance.

So he reported her missing himself to police in Brisbane.

Lisa Brookes, Ms Brookes' stepdaughter, said the family knew it was unlikely she would be found.

"He (Dr Goeden) rang me and said 'Lisa, I have some bad news to tell you'," she said. "He started crying and said, 'I've lost Carmel overboard'. He gave me a number, said he was out of credit and hung up."

Lisa said she tried to call him back but the phone number did not work.

The Courier-Mail was unable to contact Dr Goeden, who is believed to still be at sea.

Lisa said her stepmother, who once toured with the production Cats, was incredibly fit.

Whitsunday Escape owner Trevor Rees, a former business partner of Dr Goeden and Ms Brookes, said she was a very experienced sailor.

"She was a very cautious person," he said.

In an email from Dr Goeden to Ms Brookes' ex-husband, he apologised for losing her at sea.

"I'm very sorry," he wrote.

"She was a great girl and I know I'll miss her. All I can say is that I know she loved being here and that the last few weeks were wonderful for her."

Missing Persons Unit Detective Senior Sergeant Damien Powell confirmed they were looking into Ms Brookes' disappearance. "We are assisting the Department of Foreign Affairs and Interpol in their investigations," he said.

Dr Goeden's first wife, Ellen Goeden, died in January 1984, in what friends described as a "tragic accident". A newspaper report at the time said a 37-year-old woman was walking in the Barron Gorge, north of Cairns, when she slipped while crossing a waterfall, falling 10m to her death.