The Mineral Policy Institute says plans to extend the lifetime of the Ok Tedi mine in Papua New Guinea's Western Province have been tabled far too late.
The mine is heading towards an orderly closure in 2013 when it has exhausted the copper and gold deposit at its Mt Fubilan pit.
But its operators say there's an opportunity to extend the mine's life to the year 2020 which would provide an additional 1.8 billion US dollars to state revenues.
However, extension would mean a higher rate of mine tailings dumped into local rivers.
The Institute's Charles Roche says that Ok Tedi Mining Limited says it's strange for extension plans to be emerging now.
"Given that the mine's expected to close in 2013, they really have left it too late to be looking at such a major expansion. If this was a realistic option, it should have been presented some time ago. And it now seems that the proponents of the idea are trying to push it through the PNG government too quickly without really looking at what the impacts will be on the river and the people who live on the river."