The Samoa Observer reports FAST chairman La'auli Leuatea Schmidt said on Friday that party lawyers are set to file the application on Monday.
At the close of the final count for by-election votes last Friday, Electoral Commissioner Faimalomatumua Mathew Lemisio, invoked the provision allowing two extra women members to be elected based on percentage of votes cast in the constituency for them.
The Head of State has now issued writs of appointment for the two HRPP members Aliimalemanu Alofa Tu'uau and Fa'agaseali'i Sapo'a Feagia'i.
The newspaper report said FAST party advisors say that for Faimalomatumua to go ahead on both counts to issue election orders signed by the Head of State is illegal and will be questioned in court.
"We will file a motion on Monday," said La'auli.
"The caucus unanimously agreed on Thursday night to challenge the appointment of the two women added by the Office of the Electoral Commissioner," he said.
Meanwhile, Faimalomatumua, a lawyer and journalist by profession, resigned with immediate effect on Wednesday to take up the role of deputy President of the Land and Titles Court.