Multiple fire crews were called to the NSW South Coast town of Bomaderry around 3.10am, finding a rear garage and the back section of the home well alright.

Despite only taking fire crews around 15 minutes to bring the blaze under control, the garage and the back section of the home was destroyed in the blaze.

But it was the actions of a seven-year-old girl, who raised the alarm by alerting her family to the danger, that is being praised.

Nowra Fire Captain John Dun said the young girl awoke to the noise of the rear garage alight.

"The smoke hadn't reached the inside of the house and therefore at that stage had not activated the smoke alarms," he said.

"The girl, who showed incredible bravery, raised the alarm with her parents and they along with her 13-year-old sister were able to safely escape the blaze."

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The family managed to self-evacuate before the fire crews arrived.

"This is a great example of a very young person knowing what to do in such an emergency and acting on that knowledge," Captain Dun said.

Captain Dun commended the girl for her actions.

"The smoke detectors did activate once smoke and the fire entered the home, but the young girl's actions in raising the alarm early and knowing something wasn't right and what to do about it ultimately saved the family," he said.

"At least it gave them enough time to safely escape the fire.

"She is only very young and she did a great job."

Captain Dun also praised the family, who had obviously educated their children on what to do in the event of a fire, and had a fire escape plan.

When crews first arrived at the property the garage was well alight, with the intense heat causing the fire to already spread into the rear of the home.

Crews from both Shoalhaven and Nowra battled the blaze and managed to bring the fire under control.

At the same time another crew deployed hose lines to protect a neighbouing property.

Despite the efforts of fire crews the garage, and a large section of the back of the home, including a bathroom and laundry and two bedrooms were severely damaged, including the young girl's bedroom, which was completely destroyed.

Berry 224 also provided back up during the incident while due to concerns over possible asbestos in the house, two Hazmat units from Shellharbour also attended the scene.

The cause of the fire at this stage is undetermined and investigations are ongoing.