Treasurer Scott Morrison and his department have refused to make the case for a cut to penalty rates, stressing they have "no opinion" on whether the change will create jobs.

Asked four times whether he believed the Fair Work Commission's decision to slash Sunday penalty rates would boost jobs, Mr Morrison deferred to the commission's ruling but would not concur.

"They made the decision. They said it would have that impact, and that's why they made that call," he said. "I'm not here to offer opinions. Everything I believe is set out in the budget."

It followed a Senate Estimates hearing in which Treasury secretary John Fraser would not offer an opinion on whether the penalty-rate cut would generate more jobs or boost overall wages growth.