"It was an opportunity to thank the firefighters both here and from overseas, and also the community not just here in NSW but also around the world, who've shown their support."

The ad rotates between two screens, starting with a message saying, "Thank you to the brave Australian and US firefighters defending Australia. And to the world for all your support."

This gives way to a wall of flames, accompanied by the line: "Imagine fighting a bush fire higher than this billboard."

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The top of the billboard is more than 20 metres high.

Wunderman Thompson's national chief creative officer Simon Langley said that after a chat with the RFS, both agreed on a bespoke approach.

"It was quite daunting to get an idea that was worthy of the site - but we thought just keeping it really simple and powerful, but also to say thank you, was the best way to go," he said.

Mr Langley said after the RFS told them of the "amazing" donation, the creative team quickly came together, pro bono, to form ideas on how to use the scale of this billboard, and the best way to say thank you.

"We just wanted to give people in Times Square an experience of how big these fires are and how brave these firefighters are," he said.

"We thought this was a good use of the sheer scale of this billboard."

Despite the worst of the fire season now over, there are still 24 fires burning around the state, with 18 yet to be contained.

Mr Allan said that, thankfully, heavy lightning strikes overnight had not had an adverse effect on fire grounds nor created new fires.

About 350 RFS volunteers remain in the field on Wednesday, with many assisting the State Emergency Service with storm-related hazards.

Silvercast have been contacted for comment.