"Former Senator Parry mentioned to me a few weeks ago that he was endeavouring to check his family's records", Senator Fifield said in statement to SBS News.
Stephen Parry, who is president of Australia's upper house, says he has contacted United Kingdom authorities to check if he holds dual citizenship because of his British-born father.
Meanwhile, Nationals members are pushing for NSW senator John "Wacka" Williams to take the Senate president's role if Senator Parry leaves parliament.
'I learnt about it the same time as you did, on Tuesday.
He reminded them the junior coalition partner was responsible for winning last year's election because "the National Party didn't go backwards, we actually went forwards, we held all our seats and won one (more)".
"In the event I am found to hold British citizenship by virtue of my father's status, then I will clearly be in breach of Section 44 of the constitution and would therefore resign as president of the Senate and further resign as a Senator for the state of Tasmania, as I believe the High Court has made it abundantly clear what action is required", Parry said.
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Acting prime minister Julie Bishop said all MPs had a personal responsibility to ensure they were eligible to stand for parliament.
Earlier, Prime Minister Malcolm Turnbull said he was "disappointed" Mr Parry did not speak out sooner.
Tasmanian colleague Eric Abetz paid tribute to Senator Parry and said he was confident Mr Colbeck would "hit the ground running".
Although most lawmakers in multi-cultural Australia have been checking into their citizenship status since the crisis erupted, Parry said he had only examined his own case after the High Court's decision on Friday.
Parry will become the third government member to lose their job over dual citizenship, following the disqualifications of Barnaby Joyce and regional development minister Fiona Nash.