Fire investigators have determined that two house fires in Whanganui last week were not suspicious.

Fire and Emergency Whanganui assistant area commander Jemal Weston said a fire that significantly damaged a Quick Ave house on June 27 was possibly due to sparks from a chimney getting into a roof void.

A fire contained to two rooms and part of a roof in a Putiki Dr house on June 26 may have been caused by ashes removed from a fireplace.

"It's a good reminder that ashes should be placed in a steel bucket as that's the likely cause of that fire," Weston said.

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"These two fires show it's a good idea to have working smoke alarms which are an early warning of these types of incidents."