Labor senator Murray Watt outlined to a Senate estimates hearing on Wednesday morning at least four members of the tribunal had claimed thousands in taxpayer funds despite time sheets not being kept and discrepancies between the days worked and wages received.

One tribunal member, Vanessa Plain, received $128,132 for working 102 case days between July and November last year, including around $17,000 for superannuation.

Another, Jason Harkess, earned $253,079 for his part-time tribunal work during the 2019-20 financial year.

Senator Watt calculated the part-time members, who both have daily fees of $949 for tribunal work, would have had to work most days in that time period to receive those amounts, including weekends and public holidays.

He questioned how the two could earn the pay given their positions in other organisations.

Tony Barry, another member and former Liberal staffer, was paid $251,119 for the 2019-20 financial year.

Senator Watt said he would have had to work more than five days a week in order to receive that amount, according to his daily rate of $813, which was equivalent to 260 working days excluding superannuation.

The tribunal's registrar Sian Leathem told Senator Watt part-time members were paid according to the output of work, not the time worked.

"We remunerate the part-time members based upon what we believe is a reasonable amount of time for the work that they're undertaking for us," Ms Leathem said.

"If the member was not being effective and not completing the work, they would not be entitled to any remuneration or payment."

Ms Leathem said Mr Barry had completed 263 applications to receive his remuneration for that year.

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The members, of which three were appointed by former attorney-general Christian Porter, all had ties to the Liberal Party, working for it or its members in some capacity in recent years, Senator Watt said.

The work Ms Plain and Mr Harkess conducted for the tribunal coincided with pro bono work they provided a member of the Liberal Party to challenge the Victorian government's COVID-related public health restrictions.

Ms Leathem said she was unaware of their role in acting for the Liberal Party.

Senator Watt asked Liberal senator Amanda Stoker whether she agreed there was something inappropriate about the payment arrangements and other roles the members undertook.

"Senator Stoker, you're a minister in the government. You're responsible for public funds. You don't think there's anything a little bit fishy? ... nothing a little bit smelly there?" Senator Watt said.

Senator Stoker, who represented the government at the committee, conceded preparation time was needed for the court cases Mr Harkess and Ms Plain undertook but the tribunal members could just be diligent workers.

"It's quite common for people to work on multiple matters at a time when they're at the bar," Senator Stoker said.

"I think it is potentially consistent with a diligent and honest approach to one's practice for [Ms Plain] to have done so."

Senator Watt asked whether the government could guarantee public money was not being rorted by the tribunal's members.

"If any AAT team member were rorting their entitlements, or their ability to get paid, it would be dealt with very harshly but that is not the conclusion that can be drawn from the questions you have asked this morning," Senator Stoker said.

"I am happy to have a chat with the [AAT President Justice David Thomas] about making sure that the basis upon which these people have been paid is appropriate.

"We always want to make sure that all people are claiming within their entitlements and using taxpayer funds effectively."

Ms Leathem added the payment system was in the process of being reviewed.

"[Part-time AAT members are] engaged to provide surge capacity in the division, particularly to help tackle the backlog of student fees and applications," Ms Leathem said.

"The number of daily fees, or the proportion of daily fees, paid to these members is proportionate to the standard time it should take to finalise that type of case.

"We are currently reviewing our part-time arrangements with the aim of implementing a more consistent and less complex system."

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