The heat is taking its toll on highways as road surfaces turn to liquid.
State Highway 1 near Levin was closed for three hours on Sunday after bitumen melted, causing 3km queues.
The amount of bitumen "bleeding" on highways is worrying Transit New Zealand.
Its state highway policy manager, Steve Stewart, said:"We have
noticed over the past several years that we are getting bleeding in some areas. We have launched an investigation into it and are still waiting for results."
Because the problem only happened in summer, it could be investigated properly only during this time.
Consistently high ground temperatures caused the bitumen to bleed, but factors such as traffic volumes and what was underneath the bitumen were also important.
Mr Stewart said it would be unfair to blame roading contractors for the problem, as they worked under Transit guidelines, which might need changing.
Auckland regional highways manager Philip Sutton said bitumen bleeding was sometimes a problem in Northland.
Maintenance gangs needed to get to the section of road quickly and lay a light coating of chip seal. Roads could become dangerous once they cooled, because if vehicles had taken off the seal it made the surface slick.
Roads in the Coromandel, often under the pressure of heavy holiday traffic, also suffer from the problem.
The national investigation into the bleeding problem is also looking at Otago, where roads need to cope with extremes of heat and cold. Highways on the East Coast and Taranaki are also being checked.
