Up to 380 vehicles have had their tyres punctured after tonnes of steel shards were scattered along a major highway, halting thousands of drivers.

More than 25 kilometres of the M1 Motorway north of Sydney was closed on Friday after a truck carrying metal debris spilled its contents at about 5 am.

The shards were scattered along the southbound lanes, slashing the tyres of hundreds of vehicles driven by unsuspecting motorists.

It was a difficult task to comb through small pieces of metal that could be easily lodged in crevices and tyre treads, Roads Minister Jenny Aitchison said.

"This is an unprecedented situation," she told reporters on Friday.

"It's like trying to hoover up the highway.

"We're very lucky we haven't had a fatality."

Up to 380 cars may have been impacted, with light vehicles more vulnerable to the shards deflating the tyres, she said.

Cleaning the highway could take all day, drivers were warned.

"You imagine (750kg) of finely ground steel, which is a very sharp metal object," Transport for NSW co-ordinator-general Howard Collins told reporters on Friday.

"It's not just a case of getting a road sweeper out or guys and girls with brooms.

"We are using some magnetic devices, removing embedded metal in the highway itself."

A police investigation is also under way.

One tow truck driver said she had a busy morning transporting cars to tyre repair shops.

"It's been nonstop," Nicole of Central Coast Auto Recovery told AAP from the highway.

"I've towed six cars by myself so far, and we're taking them all to Gosford tyre shops - just lots of flat tyres."

Dozens of tow trucks have been dispatched to more than 200 drivers, NRMA spokesman Katrina Usman told AAP.

"Our patrols are able to fix (single punctures) on the side of the road," she said.

"But for cars with multiple wheels damaged, we've provided tows ... as of this morning, it was nearly two hours (to wait) for roadside assistance or a tow truck."

The trucking company behind the metal spill apologised and pledged to foot the bill for impacted drivers

"We're very sorry for all the concern," Daniel Falconer of NJ Ashton Group told 2GB.

"Thank God that no one's had a major accident or anything."

Drivers of B-double trucks were advised to park until the motorway reopened, while other drivers could use the old Pacific Highway.

With AAP.