It's emerged from the water for just long enough to be photographed - Auckland's equivalent of the Loch Ness Monster.

Sort of.

The mysterious sighting of what looks like a submarine in Auckland harbour yesterday has the Coastguard and harbourmaster scratching their heads.

In the grainy photograph tradition of Big Foot and UFOs, avid sea gazer Brad Watson managed to snap the tail end of what looks like a long, sleek craft ghosting along the waterfront at 11.30am.

Despite being in full of view of the Viaduct - probably Auckland's busiest area - no-one else reported seeing it or knew any reason it might be there.

360 Discovery Cruises, which run tours of the harbour, was stumped and even the people with the best vantage point in the city could not provide an answer.

An AJ Hackett staff member who had spent most of the day perched on the harbour bridge said she had seen nothing and if someone had, they would have heard about it.

Is it a cruel, elaborate hoax by some pesky kids?

Are the Russians attacking?

Is it a sneaky ploy by Auckland Tourism to drum up visitor numbers?