Fire and Rescue crews were called The Crescent shortly after 2am where Helensburgh firefighters saw flames coming through the roof of a two-storey home.

When crews arrived, two men were outside the property, with one man, aged in his 30s, suffering serious burns to his arm before he was airlifted to Royal North Shore Hospital while the other man was uninjured.

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Neighbours heard the commotion and rushed to help with buckets and hoses.

But despite the efforts of neighbours and fire crews, the top floor of the property was completely destroyed after it became engulfed in the blaze.

Fire and Rescue NSW's investigation unit, leading station officer, Michael Forbes said fire crews did a great job of containing the fire to the one home in about an hour.

"Crews got to work very quickly, trying to contain the fire to the top level of the two storey-house and they worked to stop the fire spreading to the neighbouring houses," he said.

"The top floor of the house has been totally destroyed. The roof has collapsed.

"There was also a gas fire coming from near the kitchen so there was a lot of things happening when crews arrived.

"They did a good job containing it."

Katrina Sicilia said she was awoken around 2am and upon looking outside her bedroom window heard people yelling and trying to extinguish the fire with a hose.

She said she could see smoke then when she went to her dining room window she saw small flames.

"I called Triple 0 and told them the house was on fire," Mrs Sicilia said. "We grabbed the kids and a few of their special keepsakes then we just ran outside.

"It was very scary. It was a shock to the system to wake up to.

"I was hysterical. It was only five minutes from seeing little flames on the upper level to high flames that burnt the top of the trees.

"The level became completely engulfed."

Ms Sicilia said the injured man apologised to them as he knew her kids were scared.

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Mr Forbes said the fire investigation unit was working with police crime scene officers to determine the origin and cause of the blaze.

"We have been told by one of the occupants, which is still to be confirmed by police, that there was some cooking happening in the kitchen and the two occupants went to sleep and discovered later that there was smoke in the house," he said.

"One of the occupants said he applied water to the oil that was cooking and that has made the fire worse and we believe that is how he got injured."

Mr Forbes said the fire had penetrated through the floor and damaged an area of the ceiling of the bottom level of the property.

"There is also extensive water damage, a few inches thick, on the ground floor because firefighters used a lot of water to try and extinguish the fire," he said.

A pet snake was also found alive inside a glass enclosure in the downstairs property.

Mr Forbes said investigators were unsure if there was a working smoke alarm in the property and reminded people to ensure they had one and to check it regularly.

He also said people should always be vigilant while cooking and should never pour water on an oil fire but should instead try to cover the blaze with a fire blanket or turn off the heat source.

A ignitable liquid detection dog and his handler were also on the scene to determine if there was a presence of an ignitable liquid used in the creation of the fire.

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