Five-year-old twins Matylda and Scarlett and their 11-year-old brother Blake could not be saved when their Singleton home went up in flames in the early hours of Wednesday morning.

Their mother, Kara Atkins, and eight-year-old sister Bayley were pulled from the inferno and survived.

They were taken to John Hunter Hospital, where Ms Atkins was in a stable condition on Wednesday night.

Bayley's condition couldn't be disclosed, but police earlier in the day said she was stable.

The tight-knit community is grief-stricken by the deaths and an online fundraiser was launched hours after the family was identified.

By 9am on Thursday, more than $116,000 had been raised.

"We cannot begin to comprehend what this family is going through," one donor wrote.

"I cried for you this morning. I've lost a daughter the same way," another wrote.

"May you find the strength you need to push through, and have the love and support that we did."

Police and fire investigators are working to establish what caused the fire but it's understood a fireplace is being considered as a potential source of the blaze.

A report will be prepared for the coroner.

Australian Associated Press