Techies at WebCentral's Brisbane-based data centre suggest they are "close to resolving" the issue, which has been given "the highest priority," according to a Melbourne IT spokesperson.
Melbourne IT said that no e-mail will be permanently lost as part of the outage.
"E-mail is being queued for delivery," the spokesperson said.
Curiously, WebCentral customers can still use their e-mail accounts to send messages, but cannot receive any.
The spokesperson told iTnews there were no network connectivity issues involved, but that the issue "is impacting our desk control interface, which is why our customers also can't access webmail."
At present the company isn't able to identify the cause of the problem.
"The focus is entirely on getting the service back online," the spokesperson said. "Once resolved we'll work on getting a clearer picture of the root cause of the problem."