A shed being used as a sleepout was damaged by fire in one of two suspicious blazes overnight.

Turangi Fire Brigade chief fire officer Tangonui Kingi said it was initially feared a woman and her grandchildren were inside the sleepout when the fire started just before 1am.

However, the woman was alone when the fire broke out and she had managed to safely get out of the burning building.

Mr Kingi said the woman woke her daughter and young grandchildren in the nearby house and made sure they were safely outside.

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He was concerned about an absence of fire alarms in the shed conversion.

He said it was "down to luck" that she managed to wake up and escape the fire.

In Auckland a derelict house in Glen Innes was damaged by fire just after 2am.

Fire crews contained the fire to the rear of the uninhabited Chiltern Crescent property.

Northern Fire shift manager Megan Ruru said fire safety officers and police would be returning to the scenes of both blazes to investigate further.

Both were being treated as suspicious.