Labor set for by-election test after High Court outs Gallagher

Labor senator Katy Gallagher has lost her seat in parliament after the High Court ruled she was ineligible to stand for the 2016 election because of her dual citizenship status. The fifth MP, Rebekha Sharkie, is part of minor party Centre Alliance.

Labor already faces a byelection for the seat of Perth, with Tim Hammond announcing last week he would resign for family reasons.

Lamb, Keay and Wilson are Labor Party MPs, while Sharkie is a conservative MP with the Nick Xenophon Team.

The High Court in Canberra unanimously ruled that the senator should have been disqualified from running and said she was not eligible to hold her seat.

Ms Gallagher issued a statement which she said respected the outcome of the High Court case, and apologised to her constituents.

Opposition Leader Bill Shorten gave a "rolled gold" guarantee past year that no Labor MPs would be caught up in the dual citizenship saga.

Senator Gallagher argued she took "reasonable steps" to renounce her British citizenship prior to the last election, but due to administrative complications, it had not been processed before nominations closed.

Lamb, who holds the highly marginal Queensland seat of Longman will have to renounce her British citizenship before she can recontest her seat.

"I think Mr Shorten has a credibility problem; yesterday I think people really saw Bill Shorten exposed".

Attorney-General Christian Porter has taken the hard line, stating that the decision was a "crisp and crystal clear clarification of the law", and called for the MPs resignation today.

The Labor line on the decision is that "the law has changed".

She added that Barnaby Joyce remained in the role of Deputy Prime Minister for some time as a known dual citizen who had taken no steps to renounce his New Zealand citizenship.

However, she did not get confirmation from the British Home Office that her renunciation had been processed until after she was elected, according to documents she disclosed to Parliament a year ago.

"Labor is correct in that every High Court decision sets a precedent of some kind", Professor Williams told AAP on Thursday.

The procedure to renounce British citizenship was simple and the requirements not onerous.