Sam Woodhead, 18, failed to return to a cattle station in central Queensland on Tuesday afternoon.
The former Brighton College student from Richmond in south-west London was "within hours of death" his mother, Claire Derry said.
Mr Woodhead was found on Friday after a rescue helicopter spotted his shorts.
Ms Derry told BBC Radio 4's Broadcasting House programme: "He's apparently in a very different condition to what he was on Friday.
"He's brown and looks very well indeed but he has some kidney problems due to the dehydration."
He had lost two stone (12kg) in the three days he was missing, she said.
"We were probably within two hours of him surviving something like that.
"The plane only saw his shorts fly into the air at the last moment as they were turning away, and they then turned back. They had very little fuel left on board."
Mr Woodhead told Radio 4 he got disorientated after going on a run.
"It started like a pretty normal run and I didn't have a set route and ended up a long way from where I thought I was.
"I knew that people would be looking for me, but I knew cars or horses would not be able to reach me.
"I turned to drinking my contact lens solution and drunk some of my own urine. It was getting pretty desperate.
"I don't think I would have lasted another half day. I was on my last legs."
He added: "The helicopter was flying pretty low and, as it went along, my shorts which I'd put on top of my SOS signal to give it some colour, flew up in the air under the propeller and they turned round and saw me."
Speaking from Longreach airport in north east Australia, Ms Derry said: "We are about to fly back to Brisbane.
"The doctor feels that in about a month he'll be properly restored to his former self."
Mr Woodhead said he is planning to continue his travels.
He was reported missing by the owner of Upshot Station, where he was working. The backpacker had been in Australia for eight days when he disappeared.
Temperatures were about 37C (100F) in the area at the time.
A British backpacker who went missing after setting out on a run in the Australian outback said he was on his "last legs" when he was found.