Emerald CFA chief Paul Yandle said 140 firefighters, a water bomber and 40 fire trucks were involved in containing the fire, believed to have started shortly before 2pm in bushland off Stewart Road.

Arson and explosives squad detectives are investigating the cause of the fire, which they believe to be deliberately lit.

Mr Yandle said the volunteer firefighters put in an excellent effort to quell the blaze, which fortunately happened before a sudden wind change.

"Everybody worked flat out and managed to knock most of it out within an hour and a half. Although our firefighters were prepared, there were heat-related issues that forced two fire brigade members to be treated, one at the scene and one more serious conveyed by ambulance to hospital."

Mr Yandle said access to the fire was an issue.

"The reserve of scrubland was fairly inaccessible, but about four hectares were affected.

"Fortunately we had the fire contained by the time the wind changed. This was quite significant, as the wind reached speeds of 80kmh.

"Wild fire under those conditions would have spread rapidly in a different direction."

Arson and explosives squad detective Leading Senior Constable Glenn Jefferey said the fire was deliberately lit and came dangerously close to Emerald.

"The fire occurred about 2.5kms northeast of the town in the middle of a 37 degree day with extremely strong northerly winds."

He said they were appealing for anyone who may have information about the fire or seen any suspicious activity in or around Stewart Road on Sunday to call Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au