On the first day of hearings by the Fair Work Commission into the bitter and protracted dispute over pay and conditions, the department's lawyers opened its case saying employees were offered "balanced" agreements with wage rises that were sustainable for the government.

The commission will arbitrate the pay and conditions of workers after hearing the department and representatives for staff in what has been described as the largest industrial relations case since Qantas grounded its aircraft in a high profile workplace dispute six years ago.

Outside the commission, the main public sector union on Monday said it didn't pick the fight with the government over pay and conditions and said the Coalition was trying to impose cuts to wages and take home pay for the department's 13,500-strong workforce.

Community and Public Sector Union secretary Nadine Flood said the department's position forced its staff into industrial action, and that despite strikes, ministers had not agreed to an outcome that recognised the role of Immigration staff.

Speaking for the department before commissioners, legal counsel Paul O'Grady was quick to reject claims it had negotiated in bad faith and was unreasonable in its dealings with staff who voted down several offers before the impasse went to arbitration by Fair Work.

"In reality, there's nothing more than a robust negotiation engaged in by participants with an entrenched view of their desired outcome," Mr O'Grady said.

Union attacks on the department's approach to bargaining were misplaced and did not assist the Fair Work Commission in its role, he said.

Mr O'Grady defended Immigration's decision to follow the government's industrial policy for public servants.

"It's a fallacy to suggest that merely because it's seeking an outcome consistent with the government policy, that it's acting unreasonably or not acting in good faith."

He told the Full Bench of the commission that the department's offer to staff balanced competing interests and provided flexibility while taking account of economic conditions.

"It pays appropriate regard to the lengthy, robust and unsuccessful process of all parties to reach an agreement through bargaining."

The hearing continues this afternoon and the lawyers for the Community and Public Sector Union will open their case on Friday.

Former senior Labor government members Wayne Swan and Craig Emerson will appear as witnesses for the CPSU.