A bright orange high-visibility vest and crash helmet were no substitute for road sense when a cyclist tangled with motorway traffic on Auckland's Newmarket Viaduct yesterday.

Drivers on the viaduct's southbound lanes - which have been narrowed while a replacement structure is built next to it - were astounded to come across the middle-aged cyclist on one of the busiest sections of the region's motorway network.

Seemingly oblivious to the thick traffic around him and a downpour of rain, the man cycled across the 690m viaduct and a further 1km to near the Market Rd offramp before police, responding to a flurry of phone calls from drivers, escorted him out of harm's way.

After pleading he was an overseas tourist who did not know cyclists were barred from New Zealand motorways, the man - who is believed to be from Australia - was let off with a warning to continue his journey south along urban streets.

Cyclists and pedestrians risk fines of $250 for being on motorways.

Landscape worker Jethro Hoskin, who photographed the cyclist as a passenger in a car which passed him just after 10am, said the man was zig-zagging in the motorway's left-hand lane when he first noticed him.

"To my amazement he kept on pedalling and continued to cycle over the overpass, completely oblivious to the traffic, not to mention the ridiculous weather conditions - I have never, ever, ever seen such a thing."

The man, whom he judged to be in his late 50s and whose socks were tucked into his trousers, appeared to be pedalling as fast as he could on a small-wheeled bike with a tent on a rear carrier.

"It looked like one of those fold-up bikes," Mr Hoskin said.

"He seemed to be giggling as people were yelling at him - he was claiming the road - he'd have to be a lunatic."

Motorways police spokesman Kevin Loughlin said the man's bike appeared well set up for touring the country.

He suggested that as an overseas visitor, the cyclist may have become "a bit disoriented".

Mr Loughlin said the police had received a number of calls from motorists expressing concern for the man's safety.

As well as carrying about 160,000 vehicles a day, the Newmarket Viaduct poses extra challenges for motorists who are separated from the construction of a $215 million replacement structure by large screens and must stay within lanes narrowed to 3.1m compared with a standard motorway width of 3.4m.

That has prompted the Transport Agency to reduce the speed limit to 70km/h until the construction project is complete.

Agency highways manager Tommy Parker said the cyclist was very lucky not to have been knocked over in such a hazardous environment.

Although cyclists and pedestrians occasionally strayed on to motorways, he said that generally happened in outer parts of the network such as around Westgate.