A government lawyer has told Australia's High Court it could not take a constitutional ban on dual citizens being elected to Parliament literally when deciding the fates of seven lawmakers.

Solicitor-General Stephen Donaghue told the High Court judges on Tuesday that five of the lawmakers, including three government ministers, should not be disqualified from Parliament for breaching the constitution because they did not voluntarily acquire or retain citizenship of another country.

The hearing it set to take three days.

Under the 116-year-old constitution, "a subject or citizen of a foreign power" is not eligible to be elected to Parliament.

The fate of Deputy Prime Minister Barnaby Joyce is most crucial to the government in an unprecedented political crisis.