One Nation senator Malcolm Roberts' High Court bid to save his job relies in part on the notion it would be "fundamentally un-Australian" to discriminate between natural-born and immigrant Australians.

The Turnbull government faces a nervous wait after the court reserved its decision on the fate of the "citizenship seven" - which includes three government MPs - after three days of hearings in Canberra. Chief Justice Susan Kiefel said the bench would endeavour to deliver its rulings "as soon as possible".

On the final day of hearings on Thursday, the justices became impatient with the arguments presented by Senator Roberts' SC Robert Newlinds, and repeatedly asked him to hurry along.

Mr Newlinds argued the court should not accept the federal government's claim that there should be a distinction drawn between those MPs born in Australia and those born overseas like his client.