The Speaker of the Nauru Parliament, Opposition MP, David Adeang, says yesterday's Supreme Court decision overturns 40 years of parliamentary practices in the country.
The Chief Justice, Robin Millhouse, declared that an earlier ruling that Parliament has the sovereign power to regulate its affairs was in error.
He said Parliament lacked a quorum when it amended the Citizenship Act two weeks ago and so the changes were invalid.
Mr Adeang had said he considered that a quorum existed and says he is seeking legal advice on the Court's decision.
"I really am having a problem accepting that one judge can overturn two judges who essentially made a judgement quite different to his as recently as 1998 and there are prevailing parliamentary practices that have served Nauru quite well in the last 40 years."