One person is dead and SH99 is closed after a serious crash near Invercargill.

A car crashed into a ditch about 5.45am on the Riverton-Wallacetown Highway. The intersection with Kennedy Rd, about 5km west of Wallacetown, is closed.

St John South Island spokesperson Ian Henderson said they sent an ambulance but the person was dead.

A police spokeswoman said a detour is in place on Waimatuku Rd, south to West Plains Rd.