Harcourts Melbourne City said it has been aware of the issue since October 24, when a third-party accessed personal information from its clients.
"We became aware that our rental property database had been accessed by an unknown third party without our authorisation," the firm said in the email.
"This platform holds certain personal information of all our landlords, tenants, trades and is used by our service."
The email said the information which was visible to the third-party included email addresses, legal names, phone numbers and copies of signatures and bank details.
The firm said they "took immediate steps to contain" the breach.
"Our commitment (is to) take privacy seriously," the email said.
"Importantly, our networks, accounts and your personal information are now all secure and our services are able to continue as normal."
Harcourts Melbourne City has been contacted by 9News for comment.