A 20-year-old man who today pleaded guilty to manslaughter was estimated to be driving at 121km/h in a 50km/h zone when he crashed, killing Invercargill nurse Emma Bagley.

The driver Dejay Rawiri Kane today pleaded guilty at the High Court at Invercargill to one charge of manslaughter and four charges of reckless driving causing injury.

Taine Rupena Tata Bryn Edwards, who was sitting in a passenger seat, was remanded without plea.

On December 7, 2018, Bagley, 37, died when the car she was in with her husband and their two children was T-boned by the car Kane was driving at the intersection of Newcastle and Clifton St in Windsor.

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He was disqualified at the time of the crash, found to be three times the legal alcohol limit and it was estimated he was driving at 121km/h.

Crown prosecutor Mary-Jane Thomas read the summary of facts to the court.

She said hours before the accident, members of the public observed the car Kane was driving - a Subaru - driving in high speed, on the wrong side of the road and weaving in and out of traffic.

A member of the public whose car was almost sideswiped by Kane's vehicle, contacted the police and followed the car at a distance to the Northern Tavern Bottle store.

He observed the driver returning a short time later to the car carrying alcohol.

After leaving the bottle store, Kane continued to drive at high speed and recklessly until they met an associate.

Kane drove the associate to his home where he consumed more beer.

At 9pm, the Subaru was again observed travelling at speed in the Newcastle street area.

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Witnesses described the sound of the vehicle accelerating and then a loud explosion, described as a "huge bang" and a sound of "metal crumpling''.

At 9.10pm, Emma Bagley was driving north a Kia car on Clifton St with her husband and their two children in the car.

As she was driving a Kia through the intersection at Newcastle and Clifton Sts her car was T-boned by Kane's car, which was travelling along Newcastle St, causing her car to be pushed through the intersection against a tree, rolling into its roof and sliding to a rest in a property.

Both defendants were trapped, while the victims were also trapped in their upside down car.

They were transported to Southland Hospital where Emma Bagley underwent several hours of surgery, but died as result of her injuries.

Her husband was stabilised before being flown to Christchurch Hospital with life threatening injuries and was in an induced coma.

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After he underwent several medical procedures before being transferred back to Southland Hospital.

Defendant Edwards had a broken arm.

Kane's blood was found to have 155mg, plus or minus seven milligrams, of alcohol per 100ml of blood in his system - three times the legal limit.

The day after the accident, a serious crash analyst examined the area and estimated the pre-crash speed of the Subaru was approximately 121km/h, while the Kia was 44km/h.

The speed limit in the area is 50km/h.

At the time of the accident, Kane was disqualified from driving for a period of seven months.

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He spoke to police and stated "he could not remember anything.''

Kane is due to be sentenced on October 15.

Edwards was remanded without plea until a case review in September.