David Cawthorn first made a complaint to Equal Opportunity Tasmania in 2016 about all three public entrances to the public square being uphill - or upstairs - from Parliament House, Salamanca and the waterfront area.

The matter then descended into a legal battle over whether the Anti-Discrimination Tribunal had the power to hear the case once developer Citta raised a constitutional defence.

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Citta argued that two of the three entrances could be considered accessible and that this adhered to Commonwealth standards, while Mr Cawthorn's argument could mean Tasmanian standards would overrule the Commonwealth - hence the constitutional question.

The full bench of the Supreme Court ruled the tribunal could hear the case, a decision which Citta appealed to the High Court.

In a unanimous decision, the High Court has ruled that the tribunal was correct in refusing to hear the case once the constitutional defence had been raised.

"The tribunal specifically found that the constitutional defence of the appellants was 'not colourable'," the decision reads.

"Whatever the merits of the constitutional defence, there is and could be no suggestion that the constitutional defence was not genuinely raised or is so incoherent as to be insusceptible of judicial determination on those merits."

Mr Cawthorn had hoped the High Court would assess the merits of his case as part of its decision, but this did not occur.

It means the only matter that has been determined is that the Anti-Discrimination Tribunal could not hear the case.

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Yet since his complaint, the tribunal has been absorbed into the Tasmanian Civil and Administrative Tribunal which can provide a magistrate to hear his case.

It is now open to Mr Cawthorn to make another disability discrimination complaint and have it heard at last, which he said he is considering.

"It is disappointing that after a five-and-a-half year fight about jurisdiction I'm now in no better position than I was in December 2016 when I filed the complaint," he said.

"Some developers will spend more money avoiding access than it would to cost to put it in.

"Now that the jurisdiction argument has been clarified, I will seriously consider a new complaint as the issue of equal access has still not been addressed."

No cost orders were made.

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Each state and the Commonwealth intervened as part of the case, given the potential implications it could have for the powers of other state-based tribunals.

Last year, Spirit Super bought the Parliament Square development for $330 million, the biggest commercial property sale in Tasmania's history. A disability access lift alongside the stairs - where Mr Cawthorn believes it could be placed - is estimated to cost $400,000.

The development had been 13 years in the making and included a new public plaza with alfresco dining and views over the waterfront.

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