UPDATED 4.01PM: A truck driver's plunge over the side of an Auckland motorway is being described as the sort of accident which "keeps you awake at night".

The truck was carrying two loads of fertiliser when it smashed through a barrier on the Southern Motorway near Spaghetti Junction this morning.

It then barrelled down at least 15 metres to another motorway underneath.

Both motorways are now reopened, after several hours of clean up crews hauling away wreckage, spilled fertiliser, and diesel.

NZTA spokesman Luigi Cappel says they were surprised at the 47-year-old driver's miraculous escape, and easily repairable gouge on the motorway below.

"We were very lucky the truck actually rolled down a bank, so he didn't actually hit any other vehicles, and a lot of the impact was absorbed by the hill," he says.