A number of Australia Post's payments and communications systems went offline after scheduled IT upgrades over the weekend had unintended consequences.
The organisation spent Monday working to restore functionality to a number of systems - including EFTPOS and Bank at Post - following the outage.
Retail outlets and stores were only able to accept cash due to failures with the electronic payments facilities.
The scheduled weekend work also affected parcel tracking, some payments transfer services such as Western Union, and SMS and email notifications for parcel lockers.
Mail and parcel deliveries were not affected.
The issue is understood to have stemmed from the last stage of AusPost's data centre migration.
Over this year's Australia Day long weekend, AusPost performed the lift and shift of its IT kit and workloads into two new state-of-the-art data centres: NextDC's M1 and Fujitsu's Noble Park facility.
"We are currently experiencing technical issues with some of our online and scanning systems," the organisation said in an advisory.
"We are working to rectify these as quickly as possible and will continue to update you as more information becomes available. We apologise for any inconvenience caused."
AusPost said electronic banking systems were restored nationally at around 9pm Monday night. It was still experiencing some delays to parcel tracking.
More to come