The car then reversed onto the road with a woman behind the wheel in her pyjamas, another woman in the passenger seat and three children under the age of 5 in the back seat. Police say one of the children was unrestrained.
"I opened the [car] door and the smell of alcohol was just incredible," Mr Mawson said. "She said to me, 'What did I hit?' She was still reversing off."
As he tried to intervene, she told him: "No, I'm fine, I'm fine. I'll get home."
"I said, 'No, you're not.' She tried closing the door, and I grabbed my phone and started dialling [police].
"She went to drive off and I looked at Dave, Dave looked at me, and I said, 'Grab those bloody keys out now.' So he quickly grabbed those out, and she was screaming at him, saying, 'I'm fine to drive.' I said to him, 'Even if you have to swallow those keys, keep a hold of them'."
Police arrived in a couple of minutes. Mr Mawson said the passenger suggested to the driver that they swap seats.
"I said, 'That's not going to help anything. We are standing here'."
The children appeared unhurt and were calm, but the sight of them in the car left Mr Mawson, a father of four, "quite gutted".
Canterbury road policing manager Inspector Al Stewart said: "Fortunately the vehicle was travelling slowly and nobody was injured.
"Some drivers just aren't getting the message. And someone knows this person is driving drunk. We need people to speak up and take action over drink drivers."
When tested, the woman had a reading of 1409 micrograms per litre of breath. She has been charged with driving with excess breath alcohol.
She was issued with an immediate licence suspension, and further charges are likely.
The issue of the children's welfare has been referred to Child, Youth and Family.
