Steve Martin has formally occupied the Senate seat vacated by his former party leader, Jacqui Lambie, who the High Court kicked out of parliament for holding a dual citizenship last year.
A recent fallout led to Ms Lambie severing ties with Mr Martin and removing him from her Jacqui Lambie Network, which no longer holds any seats.
Ms Lambie said her former colleague should have shown "loyalty" and voluntarily abandoned the seat, opening the possibility of Ms Lambie making an early return to politics.
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But Senator Martin, who is also the mayor of Devonport in Tasmania, decided to sit as an independent.
He will now be one of five crucial independent senators who will often hold the deciding vote on Turnbull government bills - along with Derryn Hinch, David Leyonhjelm, Cory Bernardi and Fraser Anning.
Senators Bernardi, Anning and Leyonhjelm have already indicated they will generally vote together as a conservative bloc.
