More than 20 outages have hit the Tax Office's IT systems in two years, prompting calls for the Coalition to stop further tech failures at the agency.
The Australian Taxation Office has told senators six incidents downed parts of its IT in 2016 before a massive outage that barred tax professionals from all of its systems for multiple days in December that year.
While the agency had previously revealed it was plagued by 10 outages in 2017, including another major, two-day tech failure in February, the ATO said in a response to a Senate estimates question on notice a further five outages troubled its systems that year.
Four of its 22 outages since 2016 stopped tax professionals reaching all of its systems, including one lasting 3.5 hours and another ending after two hours in July 2017, while other IT failures lasted from 30 minutes to eight hours.
The ATO detailed its tech problems after releasing a PricewaterhouseCoopers report last month that found the agency was unprepared for the failures hitting its systems in December 2016.
A spokeswoman said the report only related to its largest two outages, and that the Tax Office was following the recommendations it made.
Other outages had different causes, including hardware or operational problems, which had been addressed, she said.
Labor MP for Fenner Andrew Leigh and Labor's digital economy spokesman Ed Husic said the Coalition could not blame public servants, who had been left struggling after it slashed more than 3000 ATO jobs.
"It's unacceptable and the Coalition needs to take some responsibility," they said.
"They cannot go ahead with their plan for further cuts from the Australian Tax Office without explaining to the Australian people what they've done to make sure the ATO's website doesn't crash again.
"Frustrated taxpayers deserve answers."
The Tax Institute's senior tax counsel Robert Deutsch said tax professionals running smaller businesses were hardest hit by prolonged, multi-day outages, which could add costs and force them to miss deadlines.
"They need to be assured that the outages are under control," he said.
Businesses could cope with short outages, but the number occurring was concerning as more tax work was conducted electronically, Mr Deutsch said.
"The more we go down that path, the more important it is people have faith in that system and can use it on a day to day basis.
"If they lose confidence, it's a threat to the whole system."
The PwC review found the December 2016 outage had a "disproportionately significant impact" on services because such a meltdown had not been planned for, while remediation efforts might not be effective.
It said multiple component failures on a key data storage system had caused the outage and blocked applications and services being available to members of the public.
The ATO's response was hampered by the fact some control, management and monitoring systems were inaccessible because they were dependent on the hardware which had failed.
The agency said the design of its new storage hardware kept the tools related to its control, management and monitoring systems housed in a separate, independent area.
"In addition, we have implemented a real-time monitoring solution to detect and resolve issues before they have an impact on services to the community," its spokeswoman said.
Under functional and efficiency reviews established in May 2014, the government will cut $29.3 million from the ATO between 2014-15 and 2020-21.
