UPDATE 3.27PM: State Highway 1 north of Tirau has reopened following a serious crash earlier today.

"Police thank motorists for their patience," says a police spokesperson.

"Enquiries into the circumstances of the crash are underway."

EARLIER 11.47AM:

State Highway 1 is closed following a serious crash.

Police advise SH1 has been closed 3km north of Tirau, following a serious crash which has blocked the road in both directions.

Three people are seriously injured, says a police spokesperson.

"Motorists travelling toward Cambridge on SH1 should expect lengthy delays.

"Diversions will be signposted.

"The Serious Crash Unit has been advised."