Fire investigators are on the site of a fire that destroyed a sleep out in Gonville this morning.

Two fire crews were called to a Puriri St property at 7.22am.

The cause of the blaze was yet to be determined.

"We can't tell at this stage. Fire investigators are looking at it currently," Whanganui Fire Service station officer Charlie Bilby said.

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"The sleep out was extensively damaged."

Mr Bilby said someone had been living in the sleep out but it was unoccupied at the time.

There was no other damage to the house on the property or neighbouring properties.

Fire crews stayed on the scene until just before 10am when fire investigators arrived.

"It's a good reminder to everyone to have working smoke alarms. Even if you're not home it might alert the neighbours," Mr Bilby said.

There was a smoke alarm in the sleep out but it was not yet clear if it was working.