South Australia’s water minister has vowed to divest his shares in Macquarie Group to avoid a potential conflict of interest, after the company bought a substantial stake in Australia’s largest cotton farm.

Macquarie Group announced on Friday its agricultural fund had acquired a 49% interest in Cubbie station, located in the Murray-Darling Basin food bowl.

David Speirs’ 2018 parliamentary register of interests show he has an investment in Macquarie Group. The South Australia’s Legislative Assembly clerk’s office has not yet received Speirs’ 2019 disclosure, which is due by the end of the month.

The minister bought shares in Macquarie in 2015, according to his register of interests for that year. In March 2018 Speirs divested shares in other companies including Crown Resorts, Oil Search, Aurizon Limited and Iluka Resources.

Speirs, who is also environment minister, took part in a ministerial meeting in Canberra on Sunday concerning the Murray-Darling Basin plan along with his counterparts from Victoria, Queensland, New South Wales, the Australian Capital Territory and the federal government.

South Australian premier Steven Marshall (right) and water and environment minister David Speirs. Photograph: David Mariuz/AAP

Cubbie station is 93,000 hectares and the largest irrigation property in the southern hemisphere. The south-west Queensland cotton farm has a water storage capacity equal to Sydney Harbour.

In a good year, Cubbie has the capacity to grow 330,000 bales of cotton as well as wheat, barley, sorghum and corn.

Shareholder activist Stephen Mayne, who owns three shares in Macquarie and attended the company’s annual general meeting in Sydney last week, urged Speirs to disclose how many shares he owns.

“It’s a perception problem … He should immediately divest the shares,” Mayne told Guardian Australia.

Mayne said the “optics do not look good” and it could put a “cloud over his decision-making”.

It was a safe option for politicians not to invest in Macquarie Group because it was such a “huge beast” and had had lots of different interests over the years ranging from toll roads to airports and taxi licences, Mayne said.

Speirs said he was unaware of Macquarie’s Cubbie station announcement until Monday afternoon.

“Since becoming aware of the purchase I sought advice and I do not believe there is a conflict of interest,” he said. “However, to ensure there is no misunderstanding moving forward I have decided to divest these shares.”

A spokeswoman for South Australia’s premier, Steven Marshall, declined to comment.

The state’s ministerial code of conduct says: “Ministers should avoid situations in which their private interests conflict, have the potential to conflict or appear to conflict with their public duty.

“A conflict of interest does not only encompass actual or direct conflicts of interest between a minister’s public duty and private interests. A potential or perceived conflict of interest may also constitute a conflict of interest.”