Australians trying to take advantage of new tax breaks or report their income to Centrelink have been stymied as the main online portal for government services went down for several hours.
The deadline for reporting income to Centrelink has been extended to 7.30pm AEST on Friday after the outage.
The MyGov website and associated services came back up progressively from lunchtime after being offline for about four-and-a-half hours on Friday morning.
However, the Department of Human Services said some people may still have difficulty logging in while services were restored to full capacity.
General manager Hank Jongen asked that people without urgent business on the MyGov portal wait until services were fully restored.
"We understand the timing is unfortunate," he said in a statement on Friday afternoon, apologising for the outage.
"An investigation is under way to determine what caused the outage, but we have already ruled out any possibility of a cyberattack."
He said regular payments were made overnight and thus unaffected.
Earlier on Friday, the Australian Taxation Office tweeted that "some of our services (incl. the portals & our online services via myGov) are currently unavailable or experiencing slowness".
More than a million people have filed tax returns since the start of July - almost double the usual number - seeking to take advantage of recently legislated income tax cuts.
The Australian Services Union, which covers ATO workers, said that demand - combined with a new system for delivering payroll summaries - created a perfect storm.
"I don't mind blaming the tax office when they've really not done something right, but, in this case, I think it's just been everything happening at once," ASU's taxation officers' branch secretary Jeff Lapidos told AAP.
He said the tax office couldn't have known for certain that parliament would pass the tax cuts in the first week of July, and that ministers would tell taxpayers to file their returns as soon as possible.
The new single-touch payroll system means most workers must access the online system to retrieve payroll summaries for their tax returns.
But employers have until the end of July to upload the information, whereas they formerly had to provide summaries to workers within two weeks.
Mr Lapidos said the ATO had been inundated with calls from people trying to work out how to get the information.
Labor said the outage was a "monumental stuff up".
"The government needs to take responsibility for the fact that after they told everybody to get a tax return in, they completely stuffed it up so that people weren't able to get their tax return in," shadow treasurer Jim Chalmers said.