A 30km section of the M1 Motorway, north of Sydney, was closed on Friday after a truck carrying almost a tonne of metal debris spilled its contents about 5am.
The shards were flung along the southbound lanes, slashing the tyres of hundreds of vehicles driven by unsuspecting motorists.
The lanes reopened about 2pm after magnetic devices were used to remove embedded metal in the highway.
"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.
"This is the first time we've seen (an incident to) this extent ... where a highly dangerous material of metal shards has been spread for 30 kilometres."
Up to 380 cars may have been impacted, with light vehicles more vulnerable to the shards deflating tyres, Roads Minister Jenny Aitchison.
"This is an unprecedented situation," she said.
"It's like trying to hoover up the highway.
"We're very lucky we haven't had a fatality."
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 were dispatched to assist more than 200 drivers, NRMA spokeswoman Katrina Usman said.
"Our patrols are able to fix (single punctures) on the side of the road," Ms Usman told AAP.
"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."
Some drivers fed up with the long wait times abandoned their vehicles, but transport officials were contacting them, NSW Police Chief Inspector Thomas Barnes told reporters.
Trucking company NJ Ashton Group has apologised for the spill and pledged to foot the bill for impacted drivers.
Drivers of B-double trucks were advised to park until the motorway reopened, while other drivers were diverted to the old Pacific Highway.
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