Thomas Keating, 22, is co-operating with Thai police after his girlfriend, Kyabram woman Emily Collie, 20, was tragically killed when the holidaying couple's jet-skis crashed at high speed in waters off Phuket on Sunday.

Mr Keating told police that strong sunlight reflecting from the sea made it impossible for him to see the jet-ski being ridden by his girlfriend.

On Tuesday morning, the young man's sister Bree Lyon said the family were in Thailand with Mr Keating. She said "right now he is co-operating with authorities, it was an absolute accident".

He took to Facebook on Monday night to express his heartache over losing his girlfriend of about 18 months.

"I love you so much Emily and I wish I could just bring you back into my arms," he wrote, in a post accompanied by a photo of the pair kissing.

"I'm so broken and I know I'll never never be able to mend ... I'll always be your boy and you'll always be my girl! We had so many plans for our future Emmy.

"You where such an inspiration to myself and many others! Fly high up there Emmy I know you'll always be looking down on me and everyone you loved."

Photo: Facebook/@tommy.keating.5

Hidden among hundreds of "thinking of you" "love you mate" and "I'm so sorry" comments on the post, is a message from a woman who says she was at the scene on Kata Beach with Mr Keating.

"Hey Tom. You don't know me but I am the girl who was by your side at the beach yesterday," the woman wrote.

"I just wanted to say that I am so sorry for this great loss you have to go through. Can't stop thinking about it and it will be in my mind for the rest of my life. I really hoped that she was going to make it."

Mr Keating, a plumbing apprentice from Alexandra, near Eildon, suffered minor injuries when the jet-skis collided off Kata Beach late on Sunday.

Lifeguards pulled Ms Collie from the water and, with Mr Keating, tried to revive her on the beach before she was pronounced dead at a local hospital.

Thai police lieutenant Patiwat Yodkhwan told reporters on Monday: "Ms Collie's boyfriend, Mr Keating, who was driving the other jet-ski when they collided, will face a charge of reckless driving causing death."

Ms Collie attended Goulburn Valley Grammar School in Shepparton, graduating in 2014.

Principal Mark Torriero told the Shepparton News she was a compassionate and caring person.

"A happy, smiling and popular member of the school community, she went out of her way to help, encourage and support others," Mr Torriero said.

"Emily was a very dedicated and diligent student with a deep commitment to learning."

He said the entire school community offered its deepest sympathies to the Collie family.

Ms Collie's family are travelling to Phuket to collect her body. They are expected to arrive on Wednesday, according to the Phuket News.

The Department of Foreign Affairs and Trade is providing consular assistance to Ms Collie's family, as well as Mr Keating and his family.

In a statement, the department said: "The Department of Foreign Affairs and Trade is providing consular assistance to the family of an Australian woman who died in a jet ski accident in Phuket. Consular assistance is also being provided to an Australian man involved in the accident, and his family, in line with the Consular Services Charter."

With Lindsay Murdoch