A man is in a critical condition in Southland Hospital after a motorbike - possibly a farm bike - and a truck collided on Dunlop Rd, near Wallacetown.

An ambulance spokesman said the incident happened about 11.45am.

The crash happened on a rural gravel road on the outskirts of Wallacetown, about 15 minutes from Invercargill.

The truck was carrying rocks and was travelling south. It had stopped in a dip and was apparently about to negotiate a left-hand bend.

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The Police Serious Crash Unit is investigating.