A draft report into the proposed use of cyanide at a south east New South Wales gold mine has found the mine's storage facilities would be likely to leak.

The Eurobodalla and Palerang local councils engaged consultant Peter Beck of GHD to look into a proposal to use cyanide in on-site processing at the Dargues Reef Gold Mine at Majors Creek near Braidwood.

Unity Mining's chief executive, Andrew McIlwain, has said the proposed method of processing would be safe and had been done successfully at other sites.

But a staff report to the Eurobodalla Shire Council said the report raised major concerns about the storage of waste materials under Unity Mining's proposal.

The report recommended councillors not support the proposed modification to the development consent at next week's council meeting.

It also recommended the council vote to write to the Department of Planning and Environment and ask it to refuse the application.

In a draft submission to the department, the shire said the proponent was previously fined $200,000 for polluting Majors Creek during the construction of the mine.

The draft submission said the contractor's report raised concerns about mass accumulation of chemicals in the proposed tailings storage facility that would be used to store waste after processing.

It said it wasn't a matter of if the facility would leak, but when.

The council's draft letter said because the proposed mine life was eight years and the reactive life of the waste was centuries, catastrophic failure of the storage facility would more than likely occur as a result of erosion long after mining at the site had ceased.

It said this failure would release tens of thousands of tonnes of water into the drinking water catchment.

It also said it was concerned that if the use of cyanide was approved, the proponent would then seek further modifications to increase the scale of operations at the site and to process ore from other mines.

Mayor slams cyanide proposal

Eurobodalla mayor Lindsay Brown said he was very concerned about the lack of detail about the waste storage facilities in the proposal.

"There's no information on how it will be capped, there's no information on how it will be maintained and there's no information on any of the long-term strategies in place to ensure the security of the site," Cr Brown said.

"So there too much information that's not in there to allow us to make a decision.

"Leaving out information such as that does not give us a great deal of confidence in the ability of the proponent to conduct this mining activity in a safe manner."

Cr Brown said he didn't expect any of the Eurobodalla councillors to support the mine's proposal after reading the report.

"We're very concerned about it," he said.

"I know having spoken to Palerang and also just recently I spoke to Mr (John) Barilaro from the Monaro and he's not supportive at all of the current proposal."

Cr Brown said Mr Barilaro and Bega MP Andrew Constance would both be sent a copy of the council's submission.