The Commonwealth-owned energy provider and contractor WeBuild were each fined $15,000 last week for two pollution incidents in the Kosciuszko National Park over three months in 2022.
As a result of alleged poor sediment and erosion controls, a sediment plume stretched for more than two kilometres down Yarrangobilly River, while sediment-laden water from roadworks at Tantangara, a dam on the Murrumbidgee, entered Nungar Creek.
The NSW Environment Protection Authority said its officers had warned the companies beforehand.
"The Snowy Hydro project has a big role in the transition to renewable energy and keeping the lights on in NSW, but it is being built in a highly sensitive conservation area," Environment Minister Penny Sharpe said on Monday.
"National parks have the highest level of environmental protection of any land type. Great care needs to be taken to minimise any impacts. In this case, that hasn't occurred and it's unacceptable."
The authority said all industries needed to reduce their impact but industries operating in the state's most pristine environments had an even more critical role.
"The environment around these local waterways in the Kosciuszko National Park contains highly specialised plants, animals and micro-organisms and is home to a number of endangered species like the smoky mouse and the Alpine Tree Frog," regulations official Carmen Dwyer said.
"Actions like this can severely impact the environment not just now but for years to come and can be detrimental to many species.
"These incidents simply should not have occurred."
The Yarrangobilly River incident occurred in June while the Nungar Creek incident occurred in September.
No findings of guilt have been made, and Snowy Hydro and WeBuild can challenge the fines.
