A Christchurch man has described the moment he saw armed police force a car from the road and then arrest a man following the double mosque massacres in the city.

Nathan Cambus was travelling on Brougham St, a dual carriageway running east to west in the city's south, when he saw a grey station wagon "boosting it" away from police.

He watched armed police "hit the car into the kerb" and then jump out "so fast", guns drawn, he said.

"They pulled three guns, maybe four, from the car and then they pulled the man out right over the handbrake ... it was pretty scary."

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He described the man as having blond hair but couldn't hear what was being said during the arrest. The car's front wheels were still spinning, just above the bitumen, as the incident unfolded.

Cambus filmed part of the arrest, with his footage showing the moment two police officers, guns drawn, approached the car and one pulled the man onto the ground as the second officer provided cover.

As the person, clad in dark clothing, was dragged from the car Cambus' video shows one of the officers appearing to strike the man with his weapon.

All this occurred as several vehicles continued to drive past. The video is 44 seconds long.

Prime Minister Jacinda Ardern confirmed tonight four people have been arrested in relation to the massacre - three men and one woman.

Cambus can be heard laughing through the video - he didn't know at that stage about the tragedy and felt bad for his initial reaction.

"I sound like a dickhead in the video, but I didn't know people had been killed."

His thoughts were with those affected, Cambus said.

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"I feel for all the families. It must be awful what they are going through."