A father has been left stunned after a cow was shot dead on an Auckland motorway in front of his young children.

The father, who wanted to be known only as Robert, was driving his family home from their Queen's Birthday holiday when they were stopped by traffic on State Highway 1 just south of Silverdale.

Robert said they had just pulled into a line of about 20-30 cars when he saw a cow on the side of the road.

"As we got closer we could see a steer on the side of the motorway, just on the verge of the grass.

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"I noticed it was scuffing its hooves and I thought it was going to charge the cars. It looked aggravated," he said.

"All of a sudden a cop comes out from between the cars with a rifle and shot it in the head, he then walked up closer to it and shot it in the head again – in front of everyone.

"I'd literally just said to my girls 'hey, look there's a cow' and then bang - that happened. It was surreal."

Robert said he had his 3 and 4-year-old daughters in the car who saw everything.

"Afterwards my 3-year-old stuck her head in the car seat and covered her face as she was a bit scared and didn't understand."

He was shocked that they were not given any warning before the shooting.

"It was only about 25m away from where we were parked. I think there should have been a spotter or somebody behind to warn people to look away,"he said.

"I thought it was weird that someone could discharge a rifle, and twice, with no warning for anyone.

"There were a lot of families coming back from holiday that may not have ever been around firearms before, so to have one discharged within metres of their car would have been a bit shocking to them."

Robert said it was "pretty s*#t" for people to witness, but he thought it was a better outcome than the steer going through someone's windscreen.

A police spokeswoman said that just before midday on Saturday they receive a report of a cow on the shoulder of the road on SH1 at Orewa.

"Police made numerous attempts to get the cow off the road and into a paddock safely but unfortunately this not successful and the cow was shot for public safety.

"Situations like this are dynamic and fast-moving and Police are required to make decisions in a matter of seconds to ensure the public are kept safe."