A drink-driver more than five times the legal limit tooted his horn at a highway patrol then told stunned officers he didn't know he was driving a car, police say.

Victoria Police spokesman Sergeant Wayne Wilson said the Nunawading highway patrol car was parked in Whitehorse Road Box Hill about 8.25pm yesterday when a car went past.

A Victoria Police booze bus unit. Credit:Paul Rovere

"This was nothing unusual except police attention was drawn to the vehicle as the driver was tooting his horn and waving at police as he drove past," Sergeant Wilson said.

Police pulled over and breath tested the driver, who recorded a blood-alcohol reading of .278.

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The man had his licence withdrawn immediately and will face court at a later date in relation to the matter.

"When asked why he was driving like that, the man told police he was unaware he was driving a car," Sergeant Wilson said.