The man who helped get people out of a car after it was crushed by a falling tree has described the chilling moment he could hear the screams of the injured occupants.

Curtis, who doesn't want his surname published, was heading south along Auckland's Southern Motorway during Saturday night's wild storm when he noticed a tree lying on the road near Drury at about 9.45pm.

He immediately pulled over, but he was unaware there were people in a car underneath it.

"I just thought it was a tree down on the motorway so I casually walked over, and then all of a sudden I heard the screams and then I started running towards the car.

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"I was behind them on the motorway but I didn't actually see it happen, I just saw the big tree."

Curtis said he's seen "a few" car crashes but nothing like what he saw on Saturday night.

"Nothing like that ... not where a grown man was crying and screaming out for help.

"It was quite freaky."

Seven people were in the crushed car, including a 5-month-old baby. A couple were half out of a the car when he arrived.

"I ended up getting them all out and then putting them into my four-wheel-drive so it was warm [for them]."

Curtis said he was surprised more people didn't join him.

"I just couldn't get over the fact that nobody would get out of their car to help."

Off-duty firefighter Matt Lahana, joined Curtis to help move the van out from under the tree.

"I only did what any decent Kiwi guy would do and would hope someone would do the same for me," Curtis said.

Lahana told the Herald he doesn't want any thanks for his actions - including helping a seriously injured man who appeared to have suffered a serious neck injury - but his wife Vicki has labelled him her "super hero".

Lahana, wife, Vicki, and their two children were heading back to Hamilton from Auckland at about 9.30pm when the cars travelling in front of them came to a stop.

The rain was pelting down and initially it was unclear what the hold up was, he said. After a few minutes he spied a tree laying across nearly two lanes of the road, so the Hamilton fireman donned a jersey, and braved the weather to see what was happening.

"As I got around the tree I noticed a car there. There was another guy there and I said, 'oh did the tree hit the car, who's inside?', and he said, 'oh there's a guy trapped in the front'.

"I saw [victim] trapped and asked him a few basic first aid questions to assess his condition and his wife got out and she was in shock, really."

Since the dramatic rescue, Curtis has been in touch with the family, who have expressed their gratitude for helping them.

"It was quite good to see the ones that are not in hospital are well, but they're a bit shaken up. They're sort of stunned still."

Three people were taken to Middlemore Hospital after the crash, with one man in his 40s in a serious condition. Two of them remain in hospital, both are in a stable condition.

Police blamed the accident on wild weather which lashed the city and cut power to thousands of homes.

At the time of the crash, a police spokesman said a man in his 40s was in a critical condition after a large tree came down on a car on the Southern Motorway near Rosehill.

Two other from the car had moderate injuries.

The spokesman said the weather has been "extremely inclement", and power outages meant no lights were on along the motorway.

He said the tree "came straight down on top of the car". The passengers were aged from 5 months to early 50s.

The serious crash unit is investigating.