It was filed by Dunedin City Council chief executive Sandy Graham in relation to alleged disclosure of confidential information.
Independent investigator Steph Dyhrberg has been appointed to look into the complaint.
It is understood the confidential information relates to Cr Ong's disclosures about the possibility of a hotel being built beside Forsyth Barr Stadium in Dunedin.
Cr Ong revealed to news media the name of a firm that could be involved in the project, the Russell Property Group.
Dunedin Venues said subsequently the group had been engaged to further develop concept options and undertake a viability assessment.
In an email to Cr Ong this evening, Ms Graham asked for confidentiality. He then shared her email with media.
"Please note, that given the nature of the complaint, this matter is to be treated as confidential until such time as the investigator has reviewed the material," Ms Graham said.
"The reasons for confidentiality are to protect council’s legal and commercial position given the potential liability and commercial loss that has been created by your emails.
"A failure to respect confidentiality at this point in the process is likely to constitute a further breach of the code of conduct."
In an email to media, Cr Ong portrayed himself as a victim of "false attacks".
Cr Ong has previously breached the code of conduct.
This was in relation to an email about a staff member.
Councillors asked Cr Ong to consider resigning. He declined to do so.
He earlier complained of conduct by Cr John Chambers, but his complaint was dismissed as being without substance.
