One of the drivers involved in a horror two-car crash that killed seven people at Waverley had consumed synthetic cannabis that day, an inquest has heard.

The coroner's hearing into one of New Zealand's deadliest car crashes began today - almost a year on from the crash that killed seven people.

The sole survivor says she cannot remember the crash because she had also consumed synthetic cannabis that day.

Coroner Tim Scott is holding an inquest into the two-vehicle crash, which is expected to hear evidence from the sole survivor.

Seven people died in the crash on State Highway 3 just outside of the Taranaki town of Waverley on June 27 last year.

Ian Porteous, 80, his wife Rosalie, 76, his sister Ora Keene, 84, and friend Brenda Williams, 79, were travelling in one vehicle and died at the scene.

Ani Nohinohi was the sole survivor of the other car involved, and suffered extensive injuries. Her partner Jeremy Thompson, 28, her daughter Nivek Madams, 8, and eight-week-old daughter Shady Thompson died.

Nivek Madams had celebrated her 8th birthday the day before the crash.

The officer in charge of the incident, Detective Karl Reyland, said Thompson had smoked more than three cones of synthetic cannabis on the morning of the fatal crash.

Reyland said Thompson, Nohinohi and the two children were at an address in Stratford, but left following an argument between Nohinohi and another family member.

The detective said Thompson was on his learner's licence and had more than 100 demerit points to his name.

Reyland said the vehicle stopped in Hawera and Thompson smoked more synthetic cannabis before ordering McDonald's.

He said shortly after while heading south on State Highway 3, Thompson had lost control of the vehicle, crossed the centreline and collided with the other vehicle involved in the crash.

An autopsy revealed the presence of the drugs Ketamine, Fentanyl and Rocuronium in Thompson's blood.

Reylands said it is likely that all of these drugs were administered to Thompson by
medical personnel, but that Tetrahydrocannabinol (THC) was also detected in the urine.

Sole survivor 'synnied out'

Nohinohi's evidence was read to the inquest by a police officer.

She admitted she had also consumed synthetic cannabis that day and could not remember anything after stopping in Hawera to buy more drugs.

"I think it is quite likely that I had fallen asleep because I had been 'synnied out'," Nohinohi's statement said.

Questioned, by Logan Porteous, son of Ian and Rosalie, Nohinohi confirmed that she knew Thompson had consumed synthetic cannabis before driving - but she was "wasted" herself".

When Porteous asked if she realised the impact the fatal crash had on the families involved she broke down.

Nohinohi told the coroner that she thought her partner was OK to drive because he had smoked synthetic cannabis before and then driven.

McDonald's worker smelled cannabis in car

The McDonald's worker was not in court to give her statement so it was read out by Senior Constable Mark Stoud.

Stoud said the witness had observed the car smelt of cannabis and that she saw Nohinohi putting a material into a pipe and smoking it.

The witness sae a young child in the back seat of the car who was not wearing a seatbelt.

'Close sisters' died in crash

A Givealittle page set up at the time said Nivek and Shady were "close sisters and part of a caring whānau, deeply loved by their friends & family. Their tragic passing has been devastating for everyone and we are grateful for the outpouring of support from the community.

"As the new cluster of Matariki stars for those who have left us were announced these little lives sadly slid away."

Nohinohi was critically injured in the crash - she is one of six witnesses to be called at the inquest.

She has had a long road to recovery, and told Stuff while some of her scars were visible, she tried to hide others.

"When I do cry, it's not a big, big cry. I don't like other people to see me crying."

She told Stuff she remembered leaving the Hāwera township on the morning of the crash, but nothing else until waking up in hospital.

Her delivered the tragic news about her family.

"She goes 'sorry love, Jeremy and the girls are in heaven'," Nohinohi told Stuff.

"I was destroyed."

A granddaughter of Ian and Rosalie Porteous, Shay Starrenburg, said on behalf of her family following the tragedy that they were in shock.

"We are a very close family and still can't believe this has happened."

The couple had been married for 54 years and were "extremely loving family people".

At the time the Waverley crash was the worst on New Zealand roads in 13 years, and the fourth deadliest of all time.

In April this year eight people died in a crash north of Taupō - the third deadliest on record.

The worst fatal crash in New Zealand's history was recorded in 1963, when 15 people were killed in a bus crash in Northland.

In 1995, eight people died in a house bus crash in Hawke's Bay. In 2005, nine people died in a collision between a tourist van and a truck in Matamata-Piako.