Christina Beaumont, 52, was hanging out washing in the backyard of a friend's house in Springvale South when the sinkhole opened up beneath her shortly after 11.30am.

She fell into deep underground water and was forced to tread water to prevent more mud collapsing around her.

The mother-of-six was trapped in the hole for more than 20 minutes before a neighbour heard her cries for help and called triple-0 about 12.10pm.

Firefighters used ropes to help Ms Beaumont climb out of the hole. She was rescued about 12.30pm.

Ms Beaumont's daughter Melissa said her mother collapsed soon after returning home.

"After she had a shower she went white and...just collapsed," Melissa said.

"The paramedics said she was in shock ...

Ms Beaumont remained in hospital, undergoing tests, at 6pm.

Ambulance Victoria paramedic Stephanie Palamberis said Ms Beaumont landed in deep water when the ground suddenly beneath her collapsed.

"At one stage her head was under the water," Ms Palamberis said.

"The woman told us she couldn't touch the bottom of the hole, so was trying to swim and stay afloat to avoid the risk of further mud collapse."

Country Fire Authority operations officer Paul Carrigg said Ms Beaumont was fortunate to escape serious physical injury.

"This was a very unusual incident," he said.

"Fortunately the lady has escaped without injury, though she was obviously shaken up. She's a very lucky lady."

A spokeswoman for the Greater Dandenong City Council said the sinkhole formed on the site of an old well, which had not been properly filled in.

"It was an isolated incident on that particular property," the spokeswoman said.

Neighbour Sophia Gagatsias said her husband was the first to hear Ms Beaumont's screams.

"But he didn't know where the voice [came] from because it's a little hole where she fell," the woman said.

"My husband looked over the fence…and then he called me."

Ms Gagatsias she was still concerned for the woman, although she had not been seriously injured.

"I'm still in shock, still in panic, what about the good lady," she said.

Ms Beaumont, who has two grandchildren, was at the Springvale South property to help an elderly friend with her cleaning and washing.

"She visits her once a fortnight to help her around the house," Melissa said.

She said her family had been preparing to celebrate her younger sister's 11th birthday on Tuesday night.

"Now we have to break the news to her about mum," she said.