It's a mystery how a large pine log has ended up on the side of State Highway 1 but it's lucky the potentially deadly missile did not hit any motorists.

The log, about 5 metres in length, was spotted by a motorist wedged under a safety barrier on the northern side of Piano Hill.

Northland road policing boss Wayne Ewers said it appeared the log had been on the roadside since Monday.

He said it was difficult to say exactly how it had come to be there but the most likely explanation was it had come from a logging truck.

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Drivers can be issued with $600 ticket for having an insecure load.

Previously police have said that logs fell from trailers from time to time and whether a load on a truck was secure was always a focus during roadside checks.

An industry rule stipulates that the chains holding logs in place must touch each of the logs on the outside of the load.

Some companies use more than the specified number of chains to make sure logs were tied down, but it is up to drivers to make sure their loads were secure.

A driver headed towards Marsden Point in 2012 was unaware he had lost a log near Portland turnoff, then a second near Oakleigh service station.

Officers managed to stop the driver at Mata before more logs were lost.