All potential buyers were then told there were "small traces" of methamphetamine in the property, the authority said.
In November 2013 another agent arrived at the property to hold an open home on behalf of Mr Sharma, who was away at the time.
Referred to as Mrs Z in the ruling, a salesperson from another branch of Mr Sharma's agency, was also waiting to view the property with potential buyers.
The agent holding the open home said he was aware of the contamination, but did not warn Mrs Z or her clients about it until after they left the house.
Mrs Z's clients then complained. They said the should have been told about the contamination before entering the house with their two-year-old son.
Mrs Z told the authority she was not aware of any contamination and did not receive any warning from Mr Sharma, despite having spoken to him on the phone that day.
Chairperson of the authority's complaints assessment committee, Marina Neyon, said the committee was satisfied Mr Sharma had ample time to disclose information to Mrs Z but did not do so.
The authority found Mr Sharma and Real Estate Out West engaged in unsatisfactory conduct.
Mr Sharma was ordered to apologise in writing to the complainants and pay a fine of $750.
Real Estate Out West was ordered to undertake an audit of health and safety procedures for the agency.
Mr Sharma, who is also manager of the Real Estate Out West branch, told NZME. News Service that it was the responsibility of the agent to notify anyone at the house on the day of the contamination.
He said the agent told everyone that attended the open home about the contamination.
"The agent that was doing the open home on my behalf was told he was to tell everybody about the presence of [methamphetamine] and it was then up to them if they wanted to go inside.
"The agent who took the clients through actually went in without discussing anything or asking permission," he said.
"My agent that was doing the open home was clear, and told them to come outside."
Last year Mr Sharma was ordered to pay a former assistant more than $10,000 after it was found he told her to "get the f*** out" of the office.
The Employment Relations Authority found Mr Sharma's assistant to be unjustifiably dismissed, and he was ordered to pay her for lost wages and compensation.