A ROMANTIC night inside a storm water drain under a freeway was interrupted when a police dog latched onto the unsuspecting Casanova's posterior.

But don't blame the dog - he was just doing his job.

The dog "savaged" the tunnel explorer's buttock after a chase for an alleged car thief led police into the drain in Sydney's north.

Officers swarmed a lake next to the Warringah Freeway, Artarmon when a driver fled an allegedly stolen Toyota Hi Lux overnight.

The offender jumped the road's guard rail and raced into bushes surrounding the lake.

A dog squad was called to follow his scent.

Officers followed the dog deep inside a storm water drain underneath the roadway where they stumbled upon a man, and his mattress.

Barks echoed up the chamber as the canine latched onto a person, presumed by police to be the offender.

But as a man was dragged from the drain by two officers he had a bright smile on his face, clean clothes and happily followed all instructions.

When police questioned the man about his activities in the pipe under the highway he said he was trying to sleep on a mattress he had secreted under the road.

His story was backed up by his girlfriend, who emerged soon afterwards.

Officers quickly realised this was not the driver of the Hi Lux, or two homeless people.

The Daily Telegraph believes the police had apprehended two members of the notorious "Cave Clan" while searching for the alleged car thief.

And the police dog had bitten him right on the bottom.

The couple asked not to be named but said their romantic night in the tunnel had been interrupted by the canine officer.

He said the police dog bit into his right buttock and "savaged" it during the confrontation.

The "Cave Clan" is a guerrilla type of organisation which leisurely explore storm water drains, abandoned bunkers and rail tunnels in urban areas.

Police continued to search for the Hilux driver by canvassing the area around the lake.

The offender was still on the loose when the tunnel explorers were released.

Paramedics treated the man for his dog bite at the scene.