Around 30 fire stations across NSW could be temporarily shut down under the Fire and Rescue NSW plan to take stations temporarily offline if there are insufficient staff.

Affected stations include Corrimal, Balgownie, Coledale, Thirroul, Helensburgh, Scarborough, Bundeena, Picton and Nowra.

Illawarra sub-branch secretary Andrew Clark said it was difficult to predict what the changes would look like in practice.

"Previously in the northern Illawarra two stations could be taken offline at a time, separated by another station - I believe now that's extended to three," he said.

"It's going to be like swiss cheese with holes all over the place. There will be fire trucks sitting in stations without a crew."

FRNSW has denied stations will be taken offline.

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"The Industrial Relations Commission dismissed the FBEU claims that the industrial order in relation to operational readiness would threaten life or property, increase response times and jeopardise community and firefighter safety, instead declaring it consistent with FRNSW responding to changes in its technological and operating environment in a timely and effective manner," Deputy Commissioner of operations, Jeremy Fewtrell said.

"FRNSW's is able to provide rapid emergency response based on the fastest available resource, independent of a fixed fire station location. All FRNSW fire trucks have Automatic Vehicle Location (AVL) to enable the closest and most appropriate truck to respond.

"Under FRNSW's risk-based approach, a truck is only taken offline when there are more than sufficient resources in the area to respond to emergencies. Contrary to the FBEU's claims, FRNSW is not closing any fire stations."

It is not known how long trucks might be taken offline for.

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