Concerns have been raised about Stormont plans to give MLAs the power to oversee their own expenses.
Rules on assembly member allowances had previously been set by an independent body called the Independent Financial Review Panel (IFRP).
Under new plans the decisions would be made by the Assembly Commission, a Stormont body made up of representatives of the five main parties.
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Salaries and pensions for MLAs would continue to be set by an independent body under the proposals.
Alan McQuillan, who sat on the IFRP, said the panel had cut "huge waste" and the new system could undo its work.
"What we're going back to if this is passed and acted on is a system that we came in to deal with in 2011," he told the BBC.
The IFRP was set up in 2011 and its term ended in 2016, but a new panel was not subsequently appointed.
It made determinations on MLAs' salaries, pensions and allowances, which includes staff and constituency office running costs and expenses.
Mr McQuillan said the rules had been "basically close to a free-for-all" before the panel began its work.
"We spent four years looking at what members needed, listening to them, huge consultation exercises, and then putting in a framework," he said.
In a letter to MLAs yesterday, Stormont speaker Alex Maskey said the Assembly Commission has reviewed arrangements for the determination of salaries and allowances.
He said a "twin track" approach has been agreed to create a new body to set MLA salaries and expenses, while the Commission would determine allowances.
The proposal is set to be debated by MLAs on Tuesday.
"If the assembly passes this resolution to enable the Commission to develop any new determination, the Commission is mindful of the need to ensure that any provisions both comply with wider good practice and ensure value for money in the use of public resources," Mr Maskey wrote.
"Rigorous processes will continue to be in place to ensure that members demonstrate (through invoices etc) that all expenditure is in accordance with the rules regarding such allowances before they are reimbursed."
Mr Maskey added these are "complex matters" which the Commission "considered in detail".
"The Commission believes that this approach will retain the core principle of decisions on members' salaries and pensions being decided independently, while allowing some of the difficulties members have experienced in providing constituency services over recent years to be addressed," he wrote.
