Attorney-General George Brandis has declined a Senate request to release legal advice relating to retired Family First senator Bob Day.
The Senate ordered the attorney to table documents relating to the government's legal advice received ahead of launching a Court of Disputed Returns challenge to Mr Day's eligibility to run for parliament.
However, Senator Brandis said in a letter released on Tuesday it had been the long-standing practice of governments not to disclose privileged legal advice.
The practice had the support of previous Labor and coalition attorneys-general going back to Gareth Evans in 1995.
"It is essential that privileged legal advice provided to the commonwealth remain confidential," Senator Brandis wrote.
"Access by government to such confidential advice is, in practical terms, critical to the development of sound commonwealth policy and robust law-making."
Mr Day's eligibility is being challenged on the grounds he allegedly received an indirect benefit from the government in relation to his Adelaide electorate office.
The case will be heard in the High Court on Monday, alongside that against One Nation senator Rod Culleton.
Senator Culleton's eligibility to contest a West Australian senate seat at the July election is being challenged on the grounds he had a larceny conviction against his name.
Under the constitution, a person cannot sit in parliament if they have been convicted of a charge potentially attracting a jail term of a year or more.
He confirmed on his Facebook page he would represent himself in the High Court on Monday.
"What an incredible waste of money," he wrote.
"I'm a man of my word and I'll be representing myself and I got that belly fleece ready to go."
Senator Culleton told journalists earlier this month he would take a sheep's belly fleece and stick it over his head because he was a "true Australian standing up for the Australian people".
