An 84-year-old grandmother has been found alive three days after she went missing in remote bush land in WA's Great Southern region.

Patricia Byrne's medical condition is still being assessed but police said she had been communicating with her rescuers.

The alarm was raised after Ms Byrne disappeared while helping family members maintain walking tracks in the Stirling Range National Park.

Ms Byrne had not been seen since last Thursday. ( Supplied: WA Police )

She was last seen walking down a fire break on Thursday.

Temperatures reached 37 degrees Celsius in the region on Friday and Ms Byrne had no water or food.

Almost 70 emergency volunteers and police took part in the search.

Police said Ms Byrne appeared to be "weakened by the incident", and was being taken to hospital in Albany for medical treatment.

Great Southern Police Inspector Geoff Dixon said inclement weather overnight had been particularly concerning given Ms Byrne's age.

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"The weather conditions weren't ideal overnight, we had quite strong winds and rain here on the Northern part of the Stirling Ranges," he said.

"Exposure is one of those insidious things, that creeps up on you, and at 84 years old it doesn't matter how fit you are."

Police left markers in the national park in the hope Ms Byrne would find them and wait for searchers to return. ( ABC News: Mark Bennett )

Grandmother's survival 'amazing'

Bob Cooper, who runs bush survival courses, said Ms Byrne's survival was "incredible".

Sorry, this video has expired Ms Byrne's grandson says her rescue was the best Christmas present ever.

"I'd love to know the story of how she kept herself alive," he said.

"Your will to survive is something you can't lose."

Mr Cooper had not expected Ms Byrne to survive her ordeal.

Searchers left messages taped to bottles in the Stirling Ranges with the hope Patricia Byrne would find them. ( ABC News: Mark Bennett )

"I thought she wouldn't be alive after this amount of time because you need water, she must have been able to find water or not moved," he said.

"It's so important not to give up … some people who are much fitter than an 84-year-old have just given up and have perished not far from help … so it's really amazing."