"It is unclear exactly what happened and the extent of the damage to the truck is yet to be established."
Police said the driver was not reported to have been injured. "Police will be investigating and are in initial stages of inquiry."
One social media user wrote the "disgusting" behaviour was why the "car community" copped "so much heat".
"I've been to plenty of meets where the truckers don't care and just wanna get [through].
There's no excuses, sort it out if y'all care about our future with cars."
One person told the Herald the tanker hit two cars and tried to drive off.
"The event had two groups of people attending. One group attacked the tanker while the other crew defended the tanker. The attacking group is not part of the normal car scene in Hamilton."
A young woman was reportedly injured as she tried to protect the tanker driver. "Another girl was reportedly shoved into nearby scrub and was stood on.
"The milk discharge was done by someone in the video who is clearly visible, this person has since been dealt to by his father.
"The tanker driver is a little shaken but unharmed thanks to the efforts of the local Hamilton car crew and their actions defending the driver and doing their best to stop the attack."