Labor will step up its attacks on the Turnbull government over the legality of ministerial decisions made by three senior Nationals MPs, including Deputy Prime Minister Barnaby Joyce, based on new legal advice from two eminent lawyers.

The advice, prepared by respected barristers Matthew Collins QC and Matthew Albert, raises the possibility that any decisions the three made as ministers after October 20, 2016 could be open to challenge if the High Court finds they were invalidly elected.

The federal opposition last week demanded to know if the government had sought legal advice on whether the ministerial actions of Mr Joyce, Regional Development Minister Fiona Nash or former resources minister Matt Canavan might be at risk of challenge because of their High Court citizenship cases.

Opposition Leader Bill Shorten highlighted the fact that Mr Joyce, the Resources Minister, had not yet enacted gas export controls, and asked whether it was because of "the legal risk that the Deputy Prime Minister's ministerial actions might be challenged in the courts?"