The High Court has found Devonport Mayor Steve Martin is eligible to replace Tasmanian senator Jacqui Lambie.

One Nation's Kate McCulloch had challenged Cr Martin, saying his job of Mayor made him ineligible under the constitution because it was an office of profit under the crown.

But in a unanimous decision the High Court disagreed, clearing the way for Mr Martin to enter the Senate.

The decision also means the second Tasmanian Senate seat, left vacant by the former Senate president Stephen Parry, can now be filled by Liberal Richard Colbeck.

Both were named after a count back by the Australian Electoral Commission.

But Cr Martin's bid to replace Ms Lambie — who left the Senate last year after realising she was a British citizen — stalled after the challenge from the One Nation Candidate.

Earlier today, Ms MsCulloch's lawyers told the court Cr Martin's appointment would breach section 44(iv) of the constitution.

Usually the office of profit under the crown provision has applied to candidates who have failed to give up positions such as teaching or being a public servant at the time of nomination.

But Cr Martin said his position was independent of government and did not come under the crown.

Jacqui Lambie had to leave the Senate when she discovered her British citizenship. ( ABC News: Jed Cooper )

Ms McCulloch's counsel, Robert Newlinds, had argued that as a mayor of a local council in Tasmania, Martin had a direct and indirect relationship with the executive.

Mr Newlinds had said the Tasmanian Government did have influence of issues affecting Cr Martin, including tenure and pay.

But lawyers for Cr Martin, backed by the Commonwealth and Victoria argued his position was separate from the crown, and any influence over his tenure or pay could be tempered by the parliament.

Outside the court Cr Martin was delighted.

He said he still was not sure if he would give up his job as Mayor to take on his seat in the Senate.

"It would be a big undertaking if I was to do that but let's take a deep breath," he said.

"Let's be considered in what we do moving forward.

"It hasn't been done before, it has been done now, so it's about getting on with the job."

He would not be drawn on whether he would give up the job so Jacqui Lambie could re-enter the senate.

But he said the case had been a success for those in local government who want to pursue a political career elsewhere.

"You don't have to quit your job, to apply for a job without knowing about the outcome. And that's what it was all about," he said.

"We went down this line to make sure that there's a pathway for anybody who is considering from a local government perspective to stand for a federal government and now that pathway has been clarified by the High Court for the Tasmanian people."

The High Court will have to sit again to declare both candidates the winners, before they can be sworn in as senators.