The Australian Tax Office (ATO) will shut down all communications and IT systems during the Christmas break as part of maintenance upgrades to its data centres.

All IT and communications systems, including the tax agent and business portals, electronic and e-tax lodgement services, telephone services, the ATO website and internal business systems will be offline for almost three weeks, from December 24 to January 5.

ATO CIO Bill Gibson said affected users should contact the office before the cut-off date.

“The Christmas-New Year shut down will affect all tax IT and communications systems Australia-wide. We apologise for the inconvenience,” Gibson said.

Unconfirmed reports from a source close to the ATO suggest the move was triggered by a frozen air conditioning system in the department's main data centre.

The source said the servers — some of which have never been switched off — will be taken down due to overheating concerns. He said some technicians within the department were concerned the servers may not reboot on January 5.

The ATO warned customers disruption to systems and services may continue after the scheduled maintenance competition date.

An ATO spokeswoman said the shutdown is not connected to the $680 million Change Program to upgrade its core systems and will not affect the program schedule.

She said the shutdown is the "safest and most efficient way of conducting the essential building maintenance."