Papua New Guinea officials have sealed off the area around a Chinese-owned nickel plant in the north of the country after the leakage of potentially toxic slurry that has turned the ocean red.
The Mineral Resources Authority said on Thursday it had cordoned off a portion of Basamuk Bay in the north of the country, while emergency tests are carried out to determine the scale of environmental damage.
The water and coastline were dyed ochre red by the slurry, which is said to have overflowed from tanks at the Ramu Nickel refinery for almost an hour.
The multibillion facility is run by the state-owned China Metallurgical Group, which mines and processes nickel, a metal widely used in batteries, including in electric cars.
The Mineral Resources Authority said it had moved to "prevent villagers from going near or fishing within the affected area" while investigations "determine the nature of the slurry, including toxicity, pH" and whether heavy elements were discharged.
The incident is believed to have taken place at 4:30am on Saturday, but was not reported until the afternoon.
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Officials finally reached the site on Monday and they have now been joined by Mining Minister Johnson Tuke.
The probe will "establish any procedural failures on the part of the operator and necessary remedial measures will be imposed", the authority said.
"In the meantime, we are placing our efforts in safeguarding the local communities as our priority." The mine and processing plant have been deeply controversial since their inception.
The project was commissioned in 2012 and had been delayed for almost two years by a legal challenge that ended in the Supreme Court.
Local landowners had tried to block the project because waste from the plant is dumped in the ocean rather than landfill -- so-called "deep sea tailings placement".
During a meeting in Beijing last week, the China Metallurgical Group asked visiting Papua New Guinea officials to approve plans to expand production capacity.
It was not immediately clear what caused the spill, with reports that slurry pumps or a pipe may have failed.
The local provincial governor Peter Yama has called for the ministry of environment rather than the Mineral Resources Authority to take the investigative lead.