A digger is understood to have crashed into the Penrose overbridge on Auckland's Southern Motorway.

Witnesses say it happened in the southbound lanes but they are now flowing freely at about 85km/h.

They say the digger has been moved off the motorway at Mt Wellington.

Time Saver Traffic said the bridge only has minor damage but the alarm on the bridge was triggered.

Police are yet to confirm details of the incident.

The latest crash at the Penrose overbridge comes just under two months after NZTA installed a new $1.3m dollar system in an effort to prevent such accidents.

At the end of August this year, six new electric signs, were switched on as part of a system that would give drivers repeated visual warnings if sensors on either side of the motorway deemed a vehicle too tall.

Prior to today's incident, the last big crash at the overbridge, on May 9, occurred when a Higgins driver overestimated the bridge height and ploughed into it, causing the digger he was hauling to topple and block two of the three southbound lanes on the Southern Motorway.

Hours of traffic chaos ensued.

This latest crash is believed to be the fifth such strike at Penrose this year and the 40th since 2008 at the Penrose overbridge.

According to the NZTA the bridge, which has a clearance height of 4.57m, was built in 1952 and had its underside reinforced with steel due to the frequent bridge strikes. The maximum height for trucks travelling on this part of the motorway is 4.25m.

An agency spokeswoman estimated the total cost of bridge-strike damage across the whole of the city network was around $203,000 over the past seven years.