Various error pages including "unexpected" errors, 404 errors ("file or directory not found") and "technical difficulties" were reported, and several comments posted on the ATO's Facebook page also reported specific features of myTax not working, such as fields to enter work-related expenses or franking credits. Two taxpayers said they would face financial hardship if they were unable to file their returns in the next few days.

Customers of other online services integrated with the myGov portal, including those relying on Centrelink welfare payments, have also reported problems accessing services in recent days. The ongoing issues have spurred increased calls for the ATO to "bring back e-tax", the decades-old software which taxpayers download each year to complete their tax returns and lodge them online. The 2014-15 financial year is the last year e-tax is available, with the ATO shifting fully to the myTax portal, which was introduced in 2013 and is integrated with the broader online services portal myGov, for online lodgment from next year. Taxpayers need to log in with a myGov account even if they are still using e-tax this year.

The ongoing website problems are understood to be due to IT systems not being able to cope with the increased traffic to myGov and ato.gov.au as people rushed to get their tax returns in. The websites suffered similar problems around tax time last year. Internal ATO sources and readers have told Fairfax Media the tax office has no estimate on when services will be running optimally again. "When I rang the tech support desk they informed me that they had no idea when it would be fixed," one reader said. ATO chief information officer Jane King emailed staff on Friday afternoon about the ongoing "systems issues" that were still affecting "some customers". "Please understand that we are in a new digital design which was not a part of tax time last year," she wrote.

"Capacity planning and testing was completed as part of the rolling out of the new digital design, however due to the complexity of our environment, production is always the real test." University of Technology Sydney adjunct professor John Leaney, an IT systems expert, said capacity problems between a legacy back-end system and a new front-end user interface were "common", and this was the "most likely" cause of the issues. However, it was "appalling" to suggest the "real test" for capacity issues was to wait for customers to try it out live, he said. "We're not in the 1950s; we're not even in 1990s, we've learnt a lot and from what we've learnt we apply the techniques for proper capacity modelling," Leaney said. "There should have been much better testing; it's not something you should learn the hard way on a major government system."

He described the failure of the ATO to plan for increased capacity at tax time as a "dereliction of duty". An internal source said the network link connecting myTax with MyGov was a "standard connection" which couldn't handle traffic above 2500 users at any given time, and had "never passed pressure testing". Neither the ATO nor the Department of Human Services, which runs myGov, would comment specifically on this information; nor would they respond to questions about the cause of the ongoing problems, give an outline of capacity planning, or give a time frame for when services would be working optimally. Leaney said the problems were most likely a result of poor design and planning. "When you are doing that sort of design you don't have 'architecture tweaks'; you don't say you need some more bricks in a building," he said.

"It's very disappointing." In a statement, an ATO spokeswoman said the performance of online tax services was continuing to improve. "In some cases our services are still having issues and we continue to work around the clock to improve things," the spokeswoman said. "We thank people for their continued patience and apologise for the inconvenience." The spokeswoman said MyTax was a product developed and maintained by the ATO, and that it was the responsibility of the ATO, along with "all other ATO online services".

A DHS spokesperson said each agency integrated with the myGov portal "manages their own online IT accounts", and that "capacity and volume testing is conducted on myGov before any new release". Recent disruptions to Centrelink's online services and mobile apps had been resolved, the spokesperson said. The ATO is encouraging people who need to lodge their returns urgently to call 13 28 61. The e-tax software can be downloaded here. Know more? Email us.