Electronic waste dropped at government-approved recycling points at Officeworks stores was allegedly exported to developing countries, potentially breaching a global waste treaty, a non-government organisation has said.

The US-based Basel Action Network (Ban) – which embeds GPS units inside discarded electronics and monitors them – said in a report to be released on Thursday that two LCD monitors the organisation placed at Officeworks’ “Bring IT Back” drop zones in Brisbane were tracked to an area of Hong Kong’s New Territories known for “e-waste trafficking”.

One of the monitors was later “re-exported to an e-waste processing facility” in Thailand, where Ban investigators said they observed plumes of smoke from burning circuitboards falling out over local crops of rice, castor beans and mangos.

Australia, China and Thailand are signatories to the Basel convention, a global treaty that places strict controls on the export of e-waste between first world countries and non-OECD nations.

Officeworks’ managing director, Mark Ward, said the company was “disappointed in the findings of the report”.

“We’re working with our supplier, who has government accreditation, to ensure that e-waste collected in our stores is recycled safely and appropriately,” he said.

“Complying with all relevant laws and customs and maintaining the highest levels of ethics and integrity are central to how we do business and who we do business with. We would never knowingly illegally or unethically dump waste.”

The company is a participant in the Drop Zone recycling program, which is a “government-approved recycling program of the national television and recycling scheme”, according to the Drop Zone website.

A link to these “industry-run coregulatory recycling arrangements” features on the Department of Environment and Energy website.

In September and October, 2017, Ban investigators deployed 35 old computers, printers and monitors embedded with GPS trackers to drop zones in Brisbane, Sydney, Adelaide and Perth. It then monitored the trackers over the course of a year.

In Hong Kong, investigators found the site where the two exported devices ended up. It had been cleaned out, and Ban said it believed the facility was a temporary staging area where e-waste was then re-exported.

They tracked the second device to a large factory in Tambon Khao Khan Song, Thailand, where e-waste was seen being crudely broken apart, with chemical and smelting techniques used en masse to extract gold and copper.

The NGO said investigators observed sludge that resulted from chemical acid stripping being dumped into an “onsite sludge pond and this, combined with the open dumping of ashes and slags from the smelter, was certain to contaminate the groundwater”.

“The reason [exporting e-waste to developing countries without government approval is] … illegal is because of what happens in the developing world, where the management of the waste is so horrific, highly polluting and dangerous to the workers,” Jim Puckett, Ban’s founder and director, told Guardian Australia.

“That was the case we observed about a week and a half ago in Thailand.”

In addition, the United Nations-funded report said the tracking devices indicated that another five devices – which were placed at drop zones run by companies other than Officeworks – ended up in landfill in Australia.

Puckett said the Australian government should be more vigilant about drop zone recycling programs.

He said Ban had used the Department of Environment and Energy website to identify which drop zones to leave the GPS-embedded e-waste.

“I don’t think they’re checking that these companies are really doing the right thing with it,” he said.

“A lot of them ended up in landfill and then two were exported … it’s quite a lot of volume if you extrapolate the numbers.”

Puckett said if the proportions were correct – two in every 35 devices were exported – it would represent 16,000 tonnes of e-waste.

“This is hazardous waste. It shouldn’t be going to [landfill or being exported].”

• This article was amended on 13 August 2018 to add Jim Puckett’s full name and title.