Mahalia Murphy sensationally quit SAS Australia on Tuesday during a gruelling hill climb challenge against the remaining contestants.

The Rugby League star, 29, was tasked with hauling 40 litres of water in two jugs up a steep 500 metre desert sand dune under the boiling sun.

The Sydney native looked dead to the world as she began the arduous climb and was seen struggling for breath as she told host Ant Middleton she was done.

'I'm stuffed,' she told him as she handed him her armband, ending her time on the show.

Chief Instructor Ant Middleton, 43, then told her she should be proud of her outstanding effort on the series.

'Keep your head held high. You pushed way beyond your physical and mental boundaries. This is yours to take in,' he said.

'No one will understand what you've been through apart from you. You've been amazing, I've loved having you on my course. An absolute pleasure.'  

Mahalia later reflected on leaving the show and told the cameras it took courage to walk away - and she hoped she had made her mother proud.

'Life definitely does have times where you have to accept because it is out of your control and you can't make it happen. It takes courage to do that,' she said. 

It comes after Craig McLachlan was forced to quit SAS Australia on Monday after admitting his body wasn't strong enough to continue with the course.

The 58-year-old actor admitted that while he had the mental stamina needed to go all the way, physically he was struggling.

As he attempted a grueling challenge - which saw teams crawl through mud across rough terrain- an exhausted Craig suddenly dropped to his knees.

'I need to see the doc. I feel like a hot knife through f**king butter,' he said.

Ant Middleton then instructed Craig to stand up, before he continued to yell at him until he made the decision to quit.

With Mahalia's departure, only AFL commentator Abbey Holmes, Thai Cave rescue hero Craig Challen, Olympian Matthew Mitcham and The Bachelor's Tim Robards remain in play.