The Green Street unit block in Coledale has gone up in flames on four occasions frustrating both police and exhausted fire crews.

The suspicious blaze overnight on Monday is just the latest in a string of fires in the same street. Two houses have been destroyed as well as the unit block in the last month.

Oxley Detective acting Inspector Jason Darcy told the Leader the latest fire was under investigation but called for community help.

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"There was young persons that have been seen running from the last fire and we need to speak to anyone who has information on who is behind this," he said on Tuesday.

"This new fire, as well as the others, are all believed to have been deliberately lit.

"We would appeal for anyone with any information on suspicious persons or activity to please come forward."

Firefighters found the back unit engulfed in flames overnight on Monday and it had already spread into the neighbouring unit when they arrived.

"Crews quickly got to work and extinguished the fire," Fire and Rescue NSW spokesperson said.

"Police are again on scene to investigate and track down the offenders.

"This is the fourth deliberate fire at these units.

"Please call Tamworth police or Crime Stoppers if you have any information on who is starting these fires."

Fire and Rescue NSW crews were first called to the Green Street block of units to extinguish a laundry fire started in unit 3 in November.

On December 3, it went up in flames again, after neighbours called police to report smoke billowing into the air.

On December 9, fire crews battled another deliberately lit blaze, and in November, two homes were destroyed in separate suspicious fires.

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