Police say cooking is believed to be the cause of fire underneath the Marsh St overbridge near the Trinity Wharf hotel last night.
A 111 call was made at 6.42pm.
Senior Sergeant Glenn Saunders said the fire was able to be contained to an area where it was believed someone may have been living.
A fire northern communications spokesman said the fire may have started in a homeless person's residence.
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The fire was not being treated as suspicious and inquiries were continuing.