DETERMINED thieves overcame a security alarm, electric fence and security guards to make off with a unique $110,000 caravan from a WA dealer.

Police are still looking for the brand new 24ft Supreme Territory which is thought to have been towed away by a Queensland registered Nissan Patrol with a dinghy on top.

It is believed the brazen crooks hid after the burglar alarm was activated at the George Day Caravans sales yard in Maddington.

"They were very clever," marketing manager Chris Day told Caravanning News. "Despite the security system and electrified fence, the thieves climbed an overhanging tree branch and dropped a ladder into the yard."

The thieves then set about cutting through chains and removing a padlock from the gates before making off into the night.

"It's a very nice caravan and unique. It has a slide-out and two entrance doors," Chris said.

She suspected the sales yard had been cased the previous week by someone posing as a prospective buyer.

The caravan has red decals, an electric roll-out awning, 16" wheels and mesh tray on the A-frame.

The chassis number is ST5270 and VIN 6KRSUPREMBC0000174.

Anyone with information is urged to ring police or George Day Caravans on 08 9493 5557.