Traffic is crawling after truck and a motorbike crashed on Auckland's Southern Motorway this morning.

One person was seriously injured in the crash, which happened near the Panama Rd overbridge in Mt Wellington.

Police said two lanes initially blocked by the crash have since been cleared.

However, the NZ Transport Agency was warning the left lane headed northbound was now blocked.

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"Allow extra time for congestion - or consider an alternative route," the agency tweeted.

There are heavy delays in both directions and motorists are advised to avoid the area.

- More to come