Waymouth said the case showed the financial consequences of non-disclosure.
"Not only were they ordered to forfeit the commission but also pay very substantial damages to remedy the vendors' losses," Waymouth said.
The judge had clarified the extent of "the duty of loyalty " but Waymouth said agents also had a duty to buyers to act in good faith and fairly.
"Often agents lose the focus of who their principal is and what the consequential fiduciary duty involves," he said.
"Agents must learn to put their own interests for a commission a distant second behind those of the vendor and sometimes third behind those of the buyer as well as the vendor," he said.
A statement from Colliers' Auckland office said this week said it was aware of the court finding involving the sale of a property handled by its franchisee in Palmerston North.
"While the agents involved did not intend to disadvantage their client, they did fully accept the finding of unsatisfactory conduct made by the Real Estate Authority's Complaints Assessment Committee in 2014.
The client subsequently took a civil matter before the High Court.
The franchisee is reviewing its options regarding this decision.
"Colliers International takes its obligations and responsibilities under the Real Estate Agents Act seriously. Since the matter was raised in 2014, we have reviewed our compliance guidelines, and are confident robust procedures are in place to ensure similar matters do not occur in the future," the statement said.
Neither agents would comment on the case.