Gunnedah Police's Inspector Michael Wurth said he wouldn't "go so far as saying suspicious".

"Investigations are still being conducted and forensic police have examined both scenes," he said.

"Due to the damage caused by fire to one of the scenes, its structurally unstable and ... Fire and Rescue NSW will conduct a further examination of that scene."

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NSW Fire and Rescue Gunnedah firefighter Andrew Johns said it "could be any number of things" that might have started the housefires.

Crews from Gunnedah and Boggabri fought the first fire at Herbert Street at about 6pm on Sunday night, and were called a few hours later at 2am to tackle a blaze on Little Conadilly Street.

Mr Johns said crews were called back to the second property at about 9.30am on Monday morning because some parts of the house had reignited.

"The house is pretty compromised, so bits of the roof have fallen in and there can be tiny little smoldering fires you can't see," he told the NVI.

"We try and put it out the first time but it's not always possible. It's common for houses to get reignitions."

Mr Johns said both fires started at the "front right" of the houses, which was where the fire was hottest.

He said the Herbert Street blaze started on the verandah, and then had moved into the building.

"The damage wasn't that bad, but the whole house has been impacted by smoke. There are certain things we can salvage but not a whole lot," he said.

"Both properties were vacant which is a godsend, because the [Little Conadilly Street property] in particular was very hot. It's a weatherboard construction so the property was impacted very badly."

The firefighter said it was also good that smoke alarms were installed in both homes, and that they "save lives".

Investigators are appealing for anyone with information to contact police.

Anyone with information about the incidents are urged to contact Crime Stoppers on 1800 333 000 or via https://nsw.crimestoppers.com.au. Information is treated in strict confidence.

The public is reminded not to report crime via NSW Police social media pages.