An Adani company has been fined by the Queensland government for its unauthorised release of coal-laden stormwater into the sea at its Abbot Point port during a cyclone in March.

The coal port operator, which dumped water containing more than eight times the permitted amount of sediment during Cyclone Debbie on 27 March, was issued a $12,190 penalty for a technical breach on 20 July.

The company could face a multimillion-dollar fine if found guilty of causing environmental harm.

The fine did not relate to stormwater released into the neighbouring Caley Valley wetlands, which is still subject to an ongoing investigation, the department of environment and heritage protection told the Guardian in a statement.

The infringement notice was issued to Abbot Point Bulk Coal Pty Ltd, a company which its Indian-listed parent has been trying to sell to a tax haven company linked to the Adani family for more than four years.

Australian corporate records show Abbot Point Bulk Coal Pty Ltd is a subsidiary of the Indian stockmarket-listed company Adani Ports and Special Economic Zone Limited (APSEZ), more than half of which is owned by the Adani family.

However, in financial reports APSEZ said it sold its entire interest in the Abbot Point port operations to a private Singapore-based company, Abbot Point Port Holdings Pte Ltd, during the Indian financial year 2013.

The Singapore company is in turn held by Atulya Resources Limited, an Adani family-linked company registered in the tax haven of the Cayman Islands.

While APSEZ has taken the debt from buying Abbot Point off its books, the sale has never been recorded as completed. But the companies behind Adani’s proposed expansion of Abbot Point, which would boost its export capacity for its proposed Carmichael mine, are all ultimately owned by the tax haven company.

The Abbot Point operator was granted a temporary licence to discharge stormwater with up to 100mg per litre of “suspended solids” as high rainfall from Debbie lashed the north Queensland coast.

But 10 days later, the company told the department it had breached the licence with a release recording sediment at 806mg a litre.

“This stormwater was discharged to the surrounding marine waters,” the department said in a statement.

“The company’s environmental authority and [temporary emissions licence] contained strict conditions that should have been adhered to in ensuring the environment was protected, especially during extreme weather events.

“Temporary emissions licences and environmental authorities are not taken lightly by the department and there can be harsh penalties for companies that breach their approvals.”

The department said Adani had until 17 August to contest the fine in court.

Adani has rejected claims by conservationists that coal-laden water had contaminated the nearby Caley Valley wetlands and possibly coastal waters near the Great Barrier Reef marine park.

“EHP’s investigations into releases into the wetlands is ongoing,” the department said.

The department’s director-general, Jim Reeves, had previously flagged that the “serious penalties” for corporations causing environmental harm through licence breaches included fines of up to $3.8m for deliberate breaches and $2.7m for accidents.

An Australian Conservation Foundation campaigner, Basha Stasak, said Adani’s failure to abide by “a relaxed special licence from the Queensland environment department to release pollution into the ocean” showed the company “can’t be trusted”.

“If Adani can’t safely operate the Abbot Point coal terminal, how can they be trusted to build Australia’s biggest coal mine?

“This is not a company that should be trusted with our Great Barrier Reef, our clean water or the fate of the endangered black throated finch.”

Adani had been asked for comment.

