Run-off from the Dargues Reef Mine construction site's earthworks was washed into Spring Creek, a tributary of Majors Creek, following heavy rains on February 24.

Big Island Mining was issued with an immediate Clean-Up Notice for the site, requiring immediate action to ensure appropriate environmental controls are in place and are being adhered to.

EPA staff are onsite again following another overflow on Friday.

A formal investigation into the overspill on February 24 has now been launched, which director of the south branch for NSW EPA Gary Whytcross said will determine whether there was a breach in legislation.

“We've issued a clean-up notice,” he said.

“We are now conducting a formal investigation to find out what happened and why the sediment came offsite.”

Mr Whytcross said the sediment was not toxic but could potentially impact on the health of the waterways.

“If you have sediment in the water, it can impact on the little critters that live in the creek,” he said.

“A lot of the sediment will be quickly diluted and washed away… Our hope is that there won't be any residual impact on Major's Creek.”

Unity Mining, which merged with Cortona to develop the $90 million project, previously stated that sediment and water controls are compliant and the company is working to prevent spills.

- with John Thistleton