Seven people have been injured, four seriously, in a single-vehicle crash in Northland, police say.

The crash occurred on State Highway 1 near the Piano Hill intersection south of Hikurangi, 13km north of Whāngārei, about 1.20pm.

"Initial indications are that a number of people have sustained injuries, with four reported to be in a serious condition and three in a moderate condition."

The Serious Crash Unit was attending, a police spokeswoman said.

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SH1 was initially closed at Jordan Valley Rd to the north, and Apotu Rd to the south, with diversions in place.

But as of 2.30pm the state highway had reopened, police said.