"It went hand in hand with the purpose to establish a concerted approach to Trade Me listings that protected the Hamilton agencies from the risk that one would steal a march on the others by offering to absorb the Trade Me listing fee as a default setting, forcing the others to respond or risk the loss of listings," the judgment said.
"We consider the latter was a substantial purpose of the arrangement controlling the price Hamilton agencies charged for their services by restricting the field of competition between them on that element of the price."
The agencies and their directors were ordered to pay costs of $35,000 plus disbursements.
The Commerce Commission welcomed the ruling, saying it confirmed a long-standing approach taken in working out whether competitors had colluded.
"This case will inform our future work and we urge businesses and their advisers to understand the court's findings, and the scope of the prohibition against pricing collusion between competitors," chair Anna Rawlings said in a statement.
A penalty hearing for the realtors is still to be scheduled.