The Northern Territory's environment department is investigating vegetation dieback on Groote Eylandt believed to have been caused by the dewatering of mining pits on the island.
Named by Dutch explorer Abel Tasman in 1644, Groote Eylandt in the Gulf of Carpentaria is home to a manganese mine owned by South32 subsidiary GEMCO.
GEMCO has been working to remove huge amounts of rainwater that collected in its manganese pits after Cyclone Megan dropped a record 681 millimetres of rain on the island over two days in March.
The company's pumping of water from the mining pits into nearby scrub is suspected to have caused dieback in three areas of trees, an issue first identified by the company in September.
GEMCO self-reported the issue to the NT Environment Protection Authority in early October.
Mining pits on Groote Eylandt filled with water from Cyclone Megan. (Supplied)
In a statement to the ABC, a South32 spokesperson said discharging of water outside its lease was a periodic activity, undertaken with approval from the Anindilyakwa Land Council (ALC) and the NT government.
"Using drone technology, we recently detected localised impacts to vegetation in the relevant area where water had recently been discharged," the spokesperson said.
"We stopped discharging activities immediately."
The spokesperson said the company planned to monitor the area closely and "continue discussions with the ALC regarding rehabilitation or remediation that may be necessary and appropriate".
"Our findings indicate the impacts were due to the volume of water discharged to these areas," they said.
"Tests on the discharged water did not indicate the presence of hydrocarbons or other substances likely to have caused impacts to vegetation."
South32 said it had stopped dewatering in areas where it had identified vegetation dieback. (Supplied: South32)
A Department of Environment, Parks and Water Security spokesperson said it was investigating reports of dieback in areas both inside and outside GEMCO's mining tenement boundary.
"The operator has ceased dewatering activities that are potentially linked to the vegetation dieback while an investigation is underway," the department said.
"This investigation will inform the department's next steps into the incident."
The MV Anikitos was smashed into the Groote Eylandt wharf by Cyclone Megan. (Supplied)
This week, South32 reported it had received $US150 million in insurance payouts for damage caused to its GEMCO operation during Cyclone Megan.
Groote Eylandt's wharf was destroyed by a 190-metre-long bulk carrier that was smashed into the structure by 100 kilometre-per-hour winds and 4-metre seas.
The cyclone also caused damage to a critical bridge and several roads, and left the island's residents on fuel restrictions for months.
GEMCO is continuing to repair the wharf at Groote Eylandt, after it was destroyed in March. (Supplied)
In its September quarterly report, South32 said it had received $US100 million from external insurers and a further $US50 million from its captive insurance program.
"We are continuing to work with our insurers to assess the timing and value of further recoveries under property damage and business interruption policies," the quarterly report said.
South32 said it was "on track to resume production in the December 2024 quarter".