Two people are in a critical condition after a crash involving a truck and van just south of Hamilton.

Emergency crews are at the scene on State Highway 1 at the SH21 interchange at Tamahere.

The crash was near the intersection of Bollard Rd.

Police said two people suffered critical injuries.

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Diversions were in place but the road was reopened about 11am.

The circumstances of the crash were not yet known and the Serious Crash Unit is now on site.

Emergency services were called to the scene about 6.40am.

The NZ Transport Agency Waikato warned motorists in the area to expect delays.

Those heading to and from the Hamilton Airport were advised to consider using SH3 instead.