Federal Court judge Michael Lee made the comment during a preliminary hearing for the class action, which is due to start in April.

The court has also accepted refereed reports from expert witnesses that argue PFAS exposure is linked to certain cancers and that PFAS contamination is linked to a decline in property values.

Lawyer Ben Allen, who is leading the Williamtown Class Action, said the developments were particularly significant.

"The use of independent referees appointed by the court to determine often highly technical questions arising from expert evidence is an innovative attempt to shorten the length of a trial and resolve questions early that might assist the parties to negotiate a settlement," he said.

"The adoption of expert evidence by the court, it should be stressed, is not an end point to argument. At its simplest though, it means that the parties can cut through lengthy debate and cross examination on particular topics before the formal hearing commences

"Our clients in Williamtown see the ruling as highly significant, given that for years they have either been told not to worry about potential health risks from PFAS exposure, or else told that 'the science is still unknown'. The evidence now adopted by the court in this case means that we now know with certainty about the serious health and financial risks of PFAS exposure for this community and can move forward to finally hold those responsible for this travesty accountable."

But Justice Lee's comments appear at odds with the opinion of a Commonwealth Department of Health representative who told a parliamentary inquiry into PFAS contamination on Monday that there was still no proven link between exposure and ill health.

Health department official Gary Lum said authorities "actually just don't know what PFAS exposure does in any real confidence" despite huge research interest.

"We still maintain though that when it comes to exposure to PFAS chemicals there is yet to be any conclusive proof that exposure to PFAS chemicals causes a discrete or a distinct human disease," Dr Lum said.

Meanwhile the first of two days of mediation between the Department of Defence and residents from Williamtown, Oakey and Katherine got underway on Tuesday.

If mediation is not successful the case will proceed to a full hearing.

Fullerton Cove resident Lindsay Clout said affected residents were simply seeking a fair deal.

"All we want is to be compensated for what has been taken away from us," he said.

Some have interpreted comments from Prime Minister Scott Morrision last week as a sign that the government was working towards a mediated outcome.

Speaking in Katherine, Mr Morrison said the government was approaching the mediation process in good faith.

"I mean, previous governments have just kicked that can down the road," he said.

"Our government hasn't, we've stepped up and we're engaged in mediation with those communities now."

He gave a similar response to a question from Paterson MP Meryl Swanson on Tuesday.

The Senate supported a Greens motion on Tuesday that expressed a lack of confidence in the government's handling of the PFAS contamination crisis.

"This is the strongest parliamentary condemnation yet of Scott Morrison and his government's disrespect for this issue," Senator Mehreen Faruqi said.

"For over a year, this government has refused to answer repeated calls from the Senate to respond to the parliamentary committee's recommendations.

"The government has cruelly left communities in limbo for too long.

"Instead of stepping up and implementing vital recommendations of the committee, including provision of a compensation scheme and the possibility of buybacks, assistance to property owners and businesses in the affected areas, and the appointment of a coordinator-general to manage the national response, the government has been silent.

"PFAS contamination has caused serious financial and emotional harm to people and it is simply not fair. The government must stop stringing affected communities along and show them the respect they deserve."

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