Microsoft report that this issue is now resolved.

If you are still experiencing issues with your Microsoft 365 sign in, please contact our HelpDesk for assistance.

Our HelpDesk is receiving a large number of calls in relation to Microsoft 365 (Office 365) sign in issues.

This is a known issue for Australian 365 tenants. Microsoft are continuing to investigate.

Official Microsoft 365 health status can be viewed here: https://status.office.com/