The country's second-biggest internet provider, TelstraClear, says it has restored full email access to all its customers.
A fault that arose at about 5pm on Monday affected about 180,000 email accounts. TelstraClear said late on Tuesday afternoon that it had been fixed, though a backlog of emails was still being cleared.
It says it doesn't yet know what caused the fault but will investigate.
Users were able to send but not receive emails, and could not access their accounts online.
The company's call centre was swamped by customers, one of whom said TelstraClear wasn't doing enough to alert people about the problem.
Jody Wicksteed, who runs a home business, says she didn't get any updates and only found out about the company's response through Google.
