Former senators Bob Day and Rod Culleton have been asked by the government to provide evidence they can afford to pay back salary and allowances.
Both have been declared invalidly elected by the High Court.
A Senate committee was told on Monday the Department of the Senate and Finance Department have written to Mr Day and Mr Culleton telling them they have debts to the Commonwealth and asking whether they intend seeking a waiver.
The departments have also sought details of the two former senators' financial status, both of whom are declared bankrupts.
Asked by Labor senator Penny Wong whether the money was worth pursuing, Senate Clerk Richard Pye said he was able to call off his department under public service rules if it was found to cost taxpayers too much to chase.
"An understanding of their circumstances is the first step to determine whether it's uneconomical to pursue the debt," Mr Pye said.
"We haven't merely asked for their indication but we've asked for evidence of their financial position."
Mr Pye said he was aware both former senators were bankrupts, which was why information was being sought to confirm the financial circumstances.
The finance department had legal advice both had debts to the Commonwealth but he would not reveal how much.
However the debt for Mr Day went back to February 26, 2016 - the date of his ineligibility as determined by the High Court - and the debt for Mr Culleton was from election day, July 2, 2016.
Senate President Stephen Parry said the precedent was such debts "have generally been waived".
Asked whether Mr Culleton could claim to be a "former senator" given that he was never validly elected, Mr Pye said the West Australian politician had voted in the chamber and carried out the work of a senator until the High Court ruling.