Ms O'Dwyer did not respond to requests for information about the failures, which frustrated industry professionals and taxpayers trying to lodge their annual returns between February and June.

Information provided to a Senate committee, first reported by the ABC, revealed a series of previously unreported failures lasting between a few minutes and two business days. The log of outages does not include the major ATO meltdowns in December and July, which prompted widespread criticism and an admission from Tax Commissioner Chris Jordan that the ATO's reputation was being damaged.

In February, the ATO's website, online portals, Australian Business Register and business reporting channels were unavailable for two business days, while a series of outages hit users on April 19 and four separate dates in May.

A further four outages hit in June, including a three-and-a-half-hour outage of all internal and external systems on June 22.

Officials told the committee the ATO had received 18 complaints about the failures since February, as well as seven letters or emails from tax professionals and taxpayers expressing a loss of confidence in ATO systems.

"The ATO is aware of the impact that our recent IT issues have had on our clients and we are committed to delivering systems that have minimal disruption to the community.

"We manage a large and complex IT environment and given this complexity, we will occasionally encounter unplanned issues with our systems that result in a degradation, and in some cases outages, to our services.

"Our real-time monitoring aims to detect and resolve issues before they have an impact on the services that we provide to the community. The nature of these unplanned outages are varied and there is no single systemic or underlying cause of these issues," officials said.

Industry groups have called for compensation to accountants who have struggled with online failures during peak tax time.

Fairfax Media asked Ms O'Dwyer if she was satisfied with the ATO's online services, if she had discussed the repeated outages with Mr Jordan and if a new inquiry was needed. She did not respond on Wednesday.

Labor's shadow assistant treasurer Andrew Leigh and spokesman for digital economy Ed Husic called for an explanation.

"The ATO is trying as hard as it can, but it's been left struggling thanks to staffing cuts by the Coalition," the pair said.

"Just last week we read reports that the Turnbull government is planning to continue with its cuts over the next three years, climbing to almost $30 million.

"Labor has repeatedly called for an investigation into these interruptions, but Malcolm Turnbull and his team have done nothing to reassure the public. He needs to step up and ensure the Tax Office has the support it needs to do its job.

"When Australians are facing days and days of IT outages, blaming the public servants is not an option."

Last month the ATO launched an investigation into a significant failure of online systems which forced tax return systems offline at short notice.

A report on the major pre-Christmas outages blamed multinational Hewlett Packard and stressed fibre optic cables for the chaos.