PwC Australia lost track of who prepared dozens of client privilege claims that blocked the Australian Tax Office from gathering evidence for its investigations, a Senate inquiry has heard.

Earlier this week, the ATO published a timeline of the PwC scandal that confirmed years of frustration at the firm over allegedly withholding information related to multinational tax avoidance.

As part of an ongoing Senate inquiry, Greens senator Barbara Pocock had asked the firm how many times its former general counsel had prepared or been involved in legal professional privilege claims made to the ATO between 2016 and 2020.

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In response, PwC Australia said it received at least 46 notices from the ATO requiring them to produce documents and information during this period. The orders related to ATO investigations concerning PwC and, in some cases, PwC’s clients.

PwC was unable to provide a figure.

“Having regard to the number of notices PwC has received from the ATO during the relevant period, and the volume of documents produced in response to those notices, in the time available it has not been possible to identify the number of legal professional privilege claims made to the ATO between 2016 and 2020 that [its former general counsel] … either prepared or had involvement in,” the firm’s reply said.

PwC was contacted for comment.

The ATO repeatedly failed to convince the Australian federal police to launch an investigation into PwC during 2018 because it did not have “the investigative powers” to secure key evidence from the firm.

The ATO’s timeline revealed tax officials believed many of PwC’s legal privilege claims were “baseless” and they requested advice from the commonwealth director of public prosecutions about possible prosecutions for non-compliance.

A investigation was not launched because prosecutors considered it unlikely that the ATO could provide evidence to prove the invalidity of claims beyond reasonable doubt.

A review of PwC’s internal governance conducted by former ATO second commissioner Bruce Quigley, published in March 2021, described the firm’s use of privilege as “an issue of tension with the ATO in recent times”.

Quigley’s report, which has been tabled in the Senate as part of the ongoing inquiry, also documents the ATO’s historical concerns about PwC practices.

These included “the ‘rover’ model whereby tax structures – many including cross-border arbitrage – would be ‘rolled out’ to the market”. It also referred to PwC being “at the centre of a number of taxpayer alerts” issued by the ATO.

“Clients would be ‘pushed’ into legal engagements; there was insufficient attention given to the proper engagement of legal practitioners; a perception that ‘commercial purpose’ in relation to general anti-avoidance rules were manufactured by PwC Australia and not the taxpayer’s actual purpose of entering into particular transactions,” the report said.

In 2020, the ATO second commissioner Jeremy Hirschhorn told the report that “PwC Australia’s appetite for risk has probably reduced and that some of the partners who were involved in aggressive behaviour in the past may no longer be with the firm.

“He considered that the relationship between PwC and the ATO was now much more open and transparent,” the report said.