Tardy taxpayers can still lodge their returns after a 10-minute delay on the myGov website held up some people trying to file by the October 31 deadline.
Services run by the Department of Human Services, including myGov, were "affected by a short 10 minute disruption" on Monday afternoon, general manager Hank Jongen said in a statement.
"After which services were restored. We apologise to any customers who were inconvenienced," he said.
The Australian Taxation Office says returns can still be lodged on Tuesday.
"People don't need to worry: penalties won't apply for anyone who lodges their tax return tomorrow," a spokeswoman said in a statement on Monday night.