A super fund with more than one million members has warned that customers may have had their personal or financial information stolen after a post office break-in.

“The offenders were able to force entry into a back-of-house area used for sorting mail and distributing letters into PO Boxes on three occasions, and on the fourth, left immediately when deterred by additional security measures,” Australia Post said in a statement.

Mercer Super said its post office box may have been one that was affected by the break-in, along with the PO box of Virgin Money Super.

“It’s possible that letters containing members’ personal or financial information were stolen,” the super fund told customers online.

People who sent mail to Mercer Super or Virgin Money Super between July 1 and July 17, and who haven’t seen a transaction appear on their account or received a response within the expected timeframe, are urged to contact Mercer on 1800 682 525 between 8am and 7pm Monday to Friday, or Virgin Money on 1300 652 770 between 8am and 6pm Monday to Friday.

“We do not know the exact number of members that may have been impacted,” Mercer Super said.

“We can confirm that the potential impact is limited to members who sent mail via post to our Melbourne GPO box in July and have not received an acknowledgement from us.”

At this point, it is not believed anybody’s super accounts have been affected, but “enhanced” security is in place.

“Currently there is no indication that any member’s personal information has been published externally or sold as a result of this incident,” Mercer Super said.

Australia Post said it had also increased security at the Bourke Street facility, ahead of a planned move to a new custom-designed office in August.

“Australia Post has advised potentially affected customers of the unlawful access to their Post Office Boxes at the GPO Box Room and apologises for the inconvenience caused by these criminal acts,” the organisation said.

“Unlike parcels, letters are not tracked, and Australia Post is unable to identify individual mail items or boxes targeted by the offenders.”

Victoria Police confirmed they were investigating the break-ins, but nobody had yet been arrested.

Anybody with information is urged to contact Crime Stoppers on 1800 333 000.