Narrabri Fire and Rescue crews were called in to respond to a dark substance in the water, stretching about 800 metres.

The spill was spotted along the section of creek near the Collins Park Oval in Narrabri at about 1:30pm on Tuesday.

Narrabri Fire and Rescue NSW Station Commander Shane Bradford said two local firefighting units and an additional Hazmat crew were sent to the site.

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The town's State Emergency Service (SES) supplied a boat and operators to help the specialist hazardous materials technicians get close enough to assess the substance.

"It looked like an oily slick of black streaks through the water, it wasn't completely floating on top of the water, it actually looked semi merged but it wasn't mixing and it wasn't entirely floating," Mr Bradford said.

Initial testing by Narrabri Fire and Rescue showed there was nothing "nasty" like oils, hydrocarbons, acids or alkaline found in the matter.

Mr Bradford said after consultations with NSW Environmental Protection Agency (EPA) about the negative results, it was decided the EPA would take over the investigation.

He said he expects the EPA and WaterNSW will collect more samples from the affected water, which will be sent away for further testing to try and put the pieces of the puzzle together.

"We have no idea what it is, whether it's something natural happening in the ecosystem, we're not 100 per cent sure, and that's why there's further testing," Mr Bradford said.

"In 29 years living in Narrabri and 29 years being in Fire and Rescue, I haven't seen a slick like this floating through Narrabri Creek before."

The operation kept the crews busy for several hours before they handed the reins over to the EPA.

The Leader contacted the NSW EPA for comment.

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