Gunnedah's Orange Grove Road bridge sustained serious structural damage in the blaze on Wednesday night, and has been deemed unsafe to operate.

The local council's infrastructure services director Jeremy Bartlett said the forced closure would be "truly disappointing" for Gunnedah residents who rely on it daily.

"We have no choice but to close the bridge until the new one is constructed," he said.

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"This thoughtless act will continue to inconvenience drivers and has occurred at a time when our shire is already facing the major task of repairing roads damaged by the recent floods."

The well-travelled route was scheduled for a federal and state-funded $1.2 million facelift in May to make it more accessible to heavy vehicles.

The funding had been allocated through the state government's Fixing Country Bridges Program and the federal Bridge Renewal Program.

But Mr Bartlett said this premature closure would create issues around the town

"But this closure will cause problems for residents along the road, for primary producers and for school bus services," he said.

The incident is currently under investigation by Gunnedah police and anyone with information is urged to contact them.

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