Woman dies after Sydney flat fire

A woman has died after she was pulled from a fire in a granny flat in southwestern Sydney.

The 82-year-old was rescued by her son-in-law after the building on Beatham Place at Milperra caught alight on Saturday evening.

Brett Hall, 54, said he had to enter the flat multiple times before he could find his mother-in-law.

"It was unbelievable," he told the Seven Network on Sunday

"I thought I've got to go back again."

The victim, in a critical condition, was taken to Royal North Shore Hospital with serious burns and smoke inhalation after neighbours tried their best to cool her down with water.

Police confirmed on Sunday night she had died in hospital about midday.

Three men including Mr Hall were treated for smoke inhalation.

The building and a nearby garage were destroyed.

"The cause of the fire is undetermined at this stage," NSW Police said in a statement.