Gina Rinehart, Australia’s richest person, has bankrolled former Seven media executive Bruce McWilliam’s acquisition of an almost 10% stake in Southern Cross Media, which owns the Seven Network, the Triple M and Hit radio brands and West Australian Newspapers.

While the unusual arrangement – worth about $26m – does not give Rinehart a direct stake in Southern Cross, she could take control of the shares if McWilliam breaches the terms of their deed, which was published on the ASX on Wednesday.

The agreement puts the billionaire mining magnate back in the media game after a brief foray more than a decade ago when she bought a 10% stake in the Ten Network and a 15% stake in Fairfax, but exited both.

Fairfax Media, publisher of the Sydney Morning Herald and the Age, merged with Nine in 2018 to create a Australia’s largest integrated media player.

Australia’s media market is already dominated by another billionaire, Rupert Murdoch, whose News Corp Australia is headquartered in the US but owns the majority of mastheads in Australia as well as Sky News Australia.

West Australian billionaire Kerry Stokes is the largest shareholder in Southern Cross Media, which was created when his company Seven West Media merged with Southern Cross earlier this year. The company owns Perth’s West Australian, the Weekend West, thewest.com.au and digital network PerthNow.

Stokes exited as chair after the merger but his family retains a 20% share of the company.

The McWilliam/Rinehart investment represents the second-biggest slice of the Southern Cross media empire behind Stokes’ share.

Documents filed with the ASX on Wednesday reveal that Rinehart companies provided most of the money behind the recent purchase of a 9.73% stake in Southern Cross by McWilliam.

According to the documents, Hancock Prospecting and a long list of subsidiaries has a “relevant interest” in a combined 9.5% stake in Southern Cross Media. Rinehart is considered a substantial shareholder because her finance company has the power “to control the disposal of the shares” as set out in an agreement with McWilliam on 16 April, and has “voting power” worth 9.5%.

McWilliam, a former commercial director at Seven West Media and confidant of Stokes, bought up $11.2m and $14m worth in shares in recent weeks, largely financed by Rinehart’s loan.

“I believe in the company and the shares came up at a good price given what they were at before the merger,” McWilliam told News Corp when he bought a tranche of shares last month.

“They were almost half down and it’s a top TV network and strong radio network, both with national platforms.”

Since the merger, Southern Cross has reported its total revenue is down 1.5% to $1.008bn in the half year to December.

News, Nine Entertainment, Southern Cross and Australian Community Media control more than 80% of the market.

Rinehart is an influential voice in politics, calling last month for immigrants’ social media to be screened and financially backing rightwing movements, including by buying a plane for One Nation leader Pauline Hanson.

Hancock Prospecting donated almost $900,000 to rightwing political campaign outfit Advance last financial year, according to donations data released by the Australian Electoral Commission in February.