"Exorbitant" foreign laws that impose citizenship on Australians "willy nilly" and without their knowledge should not disqualify them from Parliament, lawyers for Deputy Prime Minister Barnaby Joyce have argued in the High Court.

On the first day of hearings into the "citizenship seven", Bret Walker SC - who is also representing deputy Nationals leader Fiona Nash - said section 44 of the constitution was aimed at avoiding split allegiance among MPs.

"There's no split allegiance where you're not aware of one of them," Mr Walker said. "You cannot heed a call you cannot hear."

Mr Walker said "exorbitant" foreign laws that imposed citizenship by descent - often without the knowledge of those affected - could not be seen to create any split allegiance. He maintained that Mr Joyce and Senator Nash did not know about their dual citizenship.