The Cambridge man was still trying to piece together what happened. "I don't know a lot myself now," he said.
He was in too much pain to talk about the incident further last night, said his partner Angela Rotherham, who travelled from Wellington to be by his side yesterday.
Witness Gareth Thorne was on his way to work on the North Shore when he saw the truck plunge 7m down the bank about 5.15am.
"It was frightening. It happened right in front of me," Mr Thorne said. "It reminded me of the Terminator movies - how the big truck comes crashing down."
He was astonished Mr Greig wasn't killed. "I'm so surprised the guy survived. He is so lucky."
The witness himself was also lucky. "If I had left 30 seconds later, the truck would've come down straight on top of my car and I would've been a goner."
The impact was so intense, part of the road was gouged out and had to be re-sealed.
Mr Thorne tried to alert other motorists to the danger. He quickly pulled over, reversed, put his hazard lights on, then beeped his horn to alert approaching traffic.
He called police and approached the truck, fearing the worst.
"I thought he was dead. I thought I was going to see something pretty horrific."
He saw a "tiny bit of movement" in the cab and then, within seconds, an ambulance arrived "just by absolute chance".
Yet Mr Thorne said the scene remained dangerous as cars approached around the corner at speed. It was also too dark to clearly make out the fertiliser, diesel and debris scattered on the road.
The cab was mangled and smashed into the road, with the windscreen wrecked. The second of two trailers stuck out at a 45-degree angle, while the first trailer was disjointed. Mr Thorne said four fire engines, three more ambulances and a large group of police soon arrived.
Emergency services checked the bank for any passengers who may have been thrown out the cab. But Mr Greig was alone.
Mr Thorne said it was fortunate the crash happened at a quiet time.
"If it was a busy time, I'm sure people would have gone crashing straight into him, because the road was totally blocked and it was on a blind corner.
"It was amazing to see. I certainly won't forget it any time soon," he said. "It was unbelievable. He was flying off that motorway."
A nine-hour clean-up followed. Lanes on the Southern Motorway - as well as the link road below, between the port and the Northern and Northwestern motorways - were closed as emergency services investigated the crash and cleared the wreckage.
Colleagues of Mr Greig, who has been driving for Regal Haulage for a decade, were amazed at the truckie's escape.
Regal Haulage driver manager Alan Pye said Mr Greig had been able to climb out of the cab himself after the crash, but was kept in hospital overnight for observation and an MRI scan.
"He is in a little bit of pain. They think he might have a cracked vertebra, but they're not 100 per cent sure, which may have been a result of the seatbelt," Mr Pye said.
Mr Greig had been driving for only two hours yesterday and had stopped for a rest break shortly before the crash, Mr Pye said. The company would investigate what caused the crash.
"We're just thankful that he's relatively unscathed, and we're more thankful that nobody else was injured - that's a big thing for us," he said.
Regal Haulage director Brett McHardie also credited the Volvo truck's safety features for the driver's escape.
The police serious crash unit was still investigating the incident last night, police said, and it was too early to say how the crash happened.
- additional reporting: Patrice Dougan, NZME.
