The High Court will hold a special hearing in January as part of the case considering whether former Family First senator Bob Day was validly elected.

The Court is considering whether the former Senator, who stepped down after the last election, breached the constitution in the lease arrangements for his electoral office.

In an unusual step today, Justice Michelle Gordon agreed to hear two days of evidence, with up to three possible witnesses, in the lead-up to a full court hearing in February.

The High Court usually only hears legal argument.

Most of the issues to be considered have been settled, but some questions raised by lawyers for former Labor senator Anne McEwan, who lost her seat at the 2016 poll, are still to be worked out.

Ms McEwan's barrister, Jeremy Kirk, would not tell the court who the witnesses would be, or what they would say, but agreed to share that information with Mr Day's lawyers.

Mr Kirk has also asked the court to issue a series of subpoenas, seeking documents from several sources including the former senator himself and the National Australia Bank (NAB).

Lawyers for Mr Day said his opponents were fishing, and playing catch-up on their allegations.

Justice Gordon dismissed many of Mr Kirk's requests saying they were irrelevant, or already covered by other matters to be considered by the court.

However she allowed others to proceed.

She set a strict timetable telling the lawyers to spend their summer getting the case ready for a hearing on January 23 and 24.

Day replacement hangs in balance

The validity of Mr Day's election was referred to the High Court by the Senate, amid concerns his connection to the lease of his electoral office breached section 44 of the Constitution.

It states MPs and senators cannot have a direct or indirect connection with a business which has a contract with the government.

In Mr Day's case the Commonwealth paid no rent for the office space, but did have a lease agreement with the owners of the building.

Mr Day used to own the building but sold it to associates after he was elected.

However he backed the loan his associates used to pay for the sale, which gave rise to concerns there had been a breach.

Mr Day resigned from the Senate in November, but his election remains in question.

If it was valid, he will likely be replaced by another Family First candidate.

However if his election breached the Constitution, a countback will likely be held to determine his replacement.

How a countback would be conducted is one of the questions facing the Court.

A similar case challenging the election of One Nation Senator Rod Culleton was heard earlier this month.

That case is still awaiting judgement.