All lanes on State Highway 1 southbound at Penrose have now opened following an earlier incident.
Three southbound on-ramps which had also been closed earlier to manage the flow of traffic will also be opened shortly.
A trailer carrying a digger hit the Penrose overbridge about 1.15 this afternoon.
The NZ Transport Agency says traffic remains heavy from the Central Motorway Junction southbound and is also heavy from Highbrook to Panmure northbound.
Motorists should delay their travel, avoid the area or expect long delays.
The Transport Agency is encouraging drivers to consider using the South Western Motorway (SH20) which is currently flowing well.
The overbridge was assessed by engineers and cleared to be opened to all vehicles earlier this afternoon.