A decision to mandate paid domestic violence leave in all modern employment awards has been delayed due to "unprecedented" circumstances.

The application to insert a clause for the leave was submitted to the Fair Work Commission by the Australian Council of Trade Unions (ACTU) last year, with hearings before a Full Bench of three commissioners in December.

But any final decision will now be delayed after commission president Iain Ross referred questions about the validity of the Full Bench to the Federal Court.

The Full Bench had reserved its decision but was thrown into upheaval when, in February, commission vice president Graeme Watson published his decision against the application a day before he resigned from his role.

Professor Anthony Forsyth from RMIT University's Graduate School of Business and Law said the move was "an unusual turn of events".

"Normally where you have three members of a commission, even if they disagree they issue their decisions at the same time," he said.

"What I've never seen before is where one member actually hands down that decision in advance of the decision of the other members.

"That's, as far as I'm aware, without precedent."

The Fair Work Act states that if a commissioner on a full bench of only three people becomes "unavailable" they have to be replaced, but there is no stipulation about what to do if that commissioner has already handed down their decision.

Referral to give certainty to decision

Now, the question to the Federal Court is how the Full Bench should be constituted to ensure its decision is lawful.

Professor Forsyth said there were a couple of options the Federal Court could choose to constitute the Full Bench.

"One of the options would be that the decision already made by former Vice President Watson rejecting the claim would stand and then the other two members of the original full bench would hand down their decision," he said.

"Another option would be that the decision of former Vice President Watson is disregarded and a newly constituted full bench with the other two original members and another new member would be appointed to deal with the case.

"But because there's some doubt about all that, that's why [Justice] Ross has referred it to the Federal Court, to seek clarity."

In his decision Justice Ross said one of the main factors for referring the matter was to prevent future appeals against the decision.

"The questions involve real issues of substance relating to the operation of... the [Fair Work] Act, which the commission cannot conclusively determine and on which there appears no relevant authority," he wrote.

"Potentially, the result could be that the validity of a leave entitlement in many or all modern awards was uncertain."

Justice Ross also said conflicting submissions from the ACTU and other parties, including Australian Chamber of Commerce and Industry, about how to constitute the bench led his decision.

ACTU supports referral

ACTU president Ged Kearney said while it was a disappointing delay, the union agreed with the need for certainty on the decision.

"We're disappointed it's been delayed, for sure," she said.

"But at the same time we do not want to be in a situation where the decision, or any decision of the commission is challenged down the track."

If successful all employees covered by a modern award would be entitled to 10 paid days of domestic violence leave and 2 unpaid days.

No date has been set yet for the Federal Court matter, but it would not go ahead until at least August or October when the full court sits.

After the Federal Court decision the matter will return to the Fair Work Commission for a final decision on the leave clause.