The independent tribunal responsible for reducing the Sunday penalty rate in the hospitality and retail sectors hasn't determined how many workers will be affected.
Fair Work Commission general manager Bernadette O'Neill noted Labor claims the decision will affect up to 700,000 employees, while a government analysis says it is as few as 285,000 workers.
"The bench didn't identify (precise numbers). There is a lot of contention about all of the numbers," she told a Senate hearing on Thursday.
Ms O'Neill said there was an enormous amount of variables, including how many hours employees work.
"These are really contentious numbers," she said.
"I think the only thing that you can accurately conclude is that it's a large number."
The employment department will appear before the hearing later on Thursday.
Asked by Labor to confirm no worker can in fact be better off if they are having their penalty rates cut, Ms O'Neill conceded "I think that's right".
But she stressed transitional arrangements will be key in how workers are affected.
"The bench acknowledged the impact on employees of the decision."
The commission's inquiry into weekend penalty rates in the retail, hospitality and fast food sectors was a very large undertaking, she said.
Evidence was received from 143 witnesses and through 218 submissions.
There were 6000 further contributions from members of the public in forming the 551 page judgement.
"There was extensive evidence from those who were seeking a reduction in the penalty rates and extensive evidence from those who were resisting," Ms O'Neill said.
