Locals from Chiatz village, near where the Watut and Markham rivers meet, alerted mine management of the grim discovery on Thursday and the mine's environment team since contacted PNG's Department of Environment and Conservation.
"But there is nothing that would point to the mine's involvement in this matter," Hidden Valley general manager of sustainability and external relations David Wissink said.
"The mine has agreed to provide environmental support to help identify the cause."
Hidden Valley reports the daily data on the quality of the mine's discharged water to the DEC on a weekly and monthly basis.
"There are no abnormalities in the Hidden Valley water quality data as it is well within compliance parameters," Wissink said.
"The greatest impact on the river system in the area at the moment is the Kumalu mudslide where cubic kilometres of sediment have entered the river system due to heavy rain.
"It is not known what has caused the fish deaths however such occurrences are a well-documented phenomenon throughout the world and are usually caused by a depletion of dissolved oxygen in the water."
There have been claims of a "cyanide spill" in some quarters while the villagers who blockaded the Markham river bridge want 10 million kina compensation ($A4.5 million) and an appearance by Morobe province governor Luther Wenge.
The bridge is between provincial capital Lae and the Hidden Valley mine, plus the promising Wafi-Golpu copper-gold project, which are both operated by the Morobe Mining Joint Ventures (Newcrest Mining 50%, Harmony Gold 50%).
Tailings dams are typically impractical in PNG due to high rainfall.