Australian rugby star Mahalia Murphy has opened up about a devastating personal tragedy and burned an unread letter from her family on the latest episode of SAS Australia.

The talented rugby player, who has represented her country in both union and league, laid bare the emotional impact of losing her sister when she was just 13.

Seven sports commentator Abbey Holmes asked Murphy about the heartbreaking  tragedy while the pair sat by a campfire on Monday night's episode of the show. 

'What happened to your sister... if you don't mind me asking?' Holmes asked Murphy.

Murphy didn't shy away from the sad truth: 'Oh, she had a heart attack. She was only 24.'

Holmes: 'Had a heart attack at 24. Oh s***.'

Murphy: 'Yeah, she was on a lot of medication. She came down to Sydney for like a week because she had to see the liver doctor, a liver specialist, because her liver was bleeding.

'And in that week she passed away before the appointment ... at home.'

Holmes: 'Just at home?'

Murphy: 'Yeah.'

Holmes: 'I'm so sorry. That's awful.'

Murphy: 'It sucked.'

Murphy then revealed the brutal impact her sister's tragic death had on her mother.

'My poor mum suffered heaps, like she even lost the daughter after (that) at birth ... a couple of days after birth. Dominique, her name was,' she said.

Holmes: 'What a traumatic time for your mum.'

Murphy: 'Yeah.'

Later in the same episode chief SAS instructor Ant Middleton gave the contestants handwritten letters from their families and loved ones.

Middleton said it was up to them if they wanted to read the letters, explaining that he wouldn't allow any distractions to his mindset when he was on 'operations'.

All the contestants chose to read the letters from home - except Murphy.

She decided to burn her letter in the campfire and explained afterwards that she loved her family but could be 'distracted very easily'. 

'When you lose focus, you can go off track and I want to be at the end and I will do everything I can to be at the end,' she said.

Murphy later told the others she believed the letter was from her mother and she knew it would have impacted her.