The source of bituminous material found across several central and lower North Island roads in February remains unknown following an investigation by NZ Transport Agency (NZTA).
An NZTA spokesperson says laboratory testing found the material was aged bitumen containing embedded stone rather than product from recent road-sealing work, as initially suspected.
“After a very thorough investigation, NZTA has been unable to identify the source of bituminous material along several highways and local roads across the central and lower North Island in February 2026,” the spokesperson says.
“Following laboratory testing of several samples, it was determined that the material was aged bitumen with stone embedded in the samples, which led NZTA to investigate options for a potential source.
“Unfortunately, after a comprehensive investigation, we have not been able to categorically confirm the source.”
The material was reported on state highways between Taupō and Levin, with affected operators reporting damage to tyres and other vehicle equipment.
Transporting New Zealand subsequently worked with a heavy vehicle engineer to produce guidance for removing tar from vehicles.
The industry group had considered seeking legal advice about compensation for operators whose trucks were damaged, but told TransportTalk it will not pursue the matter further because the source of the bitumen remains unknown.
Freight operator Hall’s warned customers at the time that large quantities of tar and loose asphalt were present along sections of State Highway 1 between Waiouru and the Kāpiti Coast.
It said several road users had suffered substantial tyre damage, with particularly difficult southbound conditions reported between Manakau and Peka Peka.
Police also received reports of tar deposits on State Highway 1, State Highway 4 and State Highway 49 between Waiouru and Sanson.
Palmerston North City Council identified similar material on some local roads and worked with contractor Fulton Hogan to address the problem.
