He said two Mongrel Mob members were at the Tāneatua Superette with a young child and a confrontation occurred inside between them and the rival gang.
This spilled out into the street, he said, as told by colleagues who witnessed it from across the road.
He said the Black Power members got into a vehicle and claimed they drove into the back of the man.
He said he heard "shouting, screeching tyres and a thud," and went outside to see a fleeing vehicle and a man on the road.
Following the incident, there had been an increased Mongrel Mob presence in town, he said, and there was "a lot of tension".
There had also been a heightened police presence.
He said the Mongrel Mob members appeared to be waiting for the rival gang to drive through town again.
A chase started between the two gangs, and police were in the chase as well, he said.
A staff member at Tāneatua Superette, who did not give his name, said a person was hit with a vehicle at around 11am.
He said police were reviewing store camera footage.
A police spokeswoman said staff responded to a disorder incident involving two groups of gang members on Tuhoe St, Taneatua, just after 11am today where one person in the group was "side-swiped" by a vehicle.
Police later saw a vehicle involved in the earlier disorder incident and signalled for it to stop.
The driver initially failed to stop but later pulled over and cooperated with police, she said.
Enquiries were ongoing and no arrests had been made.
It comes afterrising gang tensions in Rotorua that saw three incidents in three days, including one involving bats and bars and another in which gang members allegedly stole rival patches.
Police said at the time they were putting "the hammer down" and had impounded one vehicle and arrested six people so far - five Black Power members and one Mongrel Mob member - in relation to the spate of violence.
Meanwhile, the tit-for-tat feud between the Tribesmen and Killer Beez linked to several shootings has spread south of Auckland, with a suspicious fire at the Tribesmen gang pad in the Bay of Plenty town of Murupara in the early hours of Monday morning.