A new interpretation of native title law could have far-reaching implications.

The Federal Court has found the Native Title Registrar does not have the jurisdiction to register an Indigenous Land Use Agreement (ILUA) unless all registered Native Title Claimants listed as “named applicants” sign it.

This does not change if any of the named applicants have died.

The ruling was made after a challenge by a four Noongar people, who were listed as named applicants but did not sign an ILUA over an area covering 200,000 square kilometres stretching from Jurien Bay to Ravensthorpe, including the Perth metropolitan area.

Native Title lawyer Ben Zillmann, says the interpretation – now known as the ‘McGlade decision’ - is a significant change from the existing legal position.

He is concerned the decision will call the validity of many existing agreements into question.

It could also halt progress on new deals between aboriginal groups and the government and private sectors.

“Negotiating and signing an ILUA enables entities to undertake activities that may impact on native title,” Zillman told the ABC.

“So they've used a lot of different parties to facilitate things like the granting of mining and petroleum rights, land titles, and infrastructure rights.

“Or governments use them to settle claims and perhaps provide for the surrender of native title.

“Without an ILUA those processes won't be able to go ahead.”

Mr Zillmann said the previous position was defined in the 2010 QGC v Bygrave case, which ruled it was not necessary for all registered native title claimants to sign an ILUA.

“That decision had stood as ‘good law’ for the past six years, but in making the McGlade decision, the full Federal Court has decided the Bygrave decision was incorrect,” he said.

“The Full Federal Court has applied what it believes the Native Title Act actually says, whether or not it is a desirable outcome.

“And if there is a need to make changes to the Act, that is a matter for Federal Parliament to legislate.”

It is unclear what will happen to any already signed ILUAs, or those currently under negotiation.

“I think the status of signed agreements is the issue that is top of people's minds,” Mr Zillmann said.

“There is no doubt that before the decision, a number of ILUAs were signed and registered in the last six or seven years after the Bygrave decision.

“So there is a question over the validity of those agreements that may have been registered but not signed by all named applicants.

“Indeed agreements entered into before the QGC v Bygrave case, there's doubts over the validity of all those agreements now, and any rights or grants and so forth that have previously been made that rely on those agreements.”

If the decision is appealed in the High Court, it may take a different view.

“Then again they may not; but it needs to be recognised that the agreements that had been signed up to were generally authorised by a majority of named claimants,” Zillman said.

“And I think it's not just pastoralists, miners and local governments that would want those [prior] agreements enforced, but quite legitimately some native title groups would want that as well.

“I think it is likely there will be a ‘legislative fix’ to retrospectively give some certainty.

“And the Federal Government has done that before; one of the fundamental aspects of the Native Title Act was to validate a whole lot of things that were done before native title was recognised by the courts, including freehold land and mining title grants.”