MP Su'a William Sio was caught breaking the law, driving a Labour-branded car emblazoned with his name while talking on his mobile phone.
An angry passenger in another vehicle snapped a photo as Sio motored along a busy two-lane road in South Auckland last month.
He says Sio held the cell to his mouth for about two minutes before turning left into a side street.
The traffic infringement attracts an $80 fine and 20 demerit points.
When questioned, Sio immediately confessed and apologised, telling The Dominion Post he was running late for an event when his phone rang.
"It was a silly thing. I shouldn't have done it. I've broken the law," he said.
"I was running late. I was holding the phone to my mouth; I didn't put it to my ear. I had the loud voice on and I picked it up at a set of lights, I recall. Then the light went green.
"I was embarrassed by doing it, knowing I was in the car. If the police decide to charge I'll absolutely pay the fine. I should know better."
Parliamentary Service issues MPs with a hands-free kit, but Mr Sio had just been given a new phone and had not fitted the new equipment.
The photo was taken about lunchtime on August 24 when Mr Sio was travelling along Cavendish Drive in Manukau.
The MP did not realise he was being caught on camera.
"He was talking and talking away, holding his phone with one hand," said the person who took the photo.
"I was first alerted to the fact that it might be the MP for Mangere when, after becoming a little annoyed at the fact the driver beside me was breaking the law, I noticed that he had his name plastered all over the car."
A police spokesman said: "Driving requires our full attention and the use of cellphones and other handheld devices while driving is a significant distraction that risks the safety of other road users.
"If the person who took the picture wishes to provide more information to police, or Mr Sio presents himself at a police station, then the matter can be dealt with."
Acting Labour leader Grant Robertson said: "Mr Sio has acknowledged that what he did was wrong. He's apologised and he has offered to pay the fine.
"I believe that's where the matter should end, and I'm sure he won't do it again."
AA spokesman Dylan Thomsen said the chances of being involved in a crash increased four-fold while using a cellphone while driving.
Texting increases the likelihood of an accident 23-fold.
"No matter how good a driver you are, our brain's attention is finite. It is not possible to focus on two things at the same time. It means you are going to be slower to react and more likely you will miss seeing something."
AA members list it as a top frustration. "That's because they know those people aren't fully focused.
"They are putting themselves and everybody else around them on the road at risk. No call is that urgent."
AA wants a review of penalties.
- Fairfax Media