According to the summary of facts Ohlson, 30, and Ata were among 30 patched Black Power gang members who congregated at the Whakatāne Bridge to meet the procession, but the procession entered town via a different route.
"You were then part of a group that gathered in a service lane. You used the cover of bushes to throw missiles like rocks, sticks and bottles at the cars in the procession," Justice Lang said.
He said the offending had potential to "cause significant ramifications".
"You [Ata] were carrying a butcher's knife in a sheath. You explained this was a work knife carried for protection."
Some of the others involved in the January 17 events were on trial in July and will be sentenced at the end of this month, while others have already been sentenced on several different charges.