SYDNEY - The world's richest woman Gina Rinehart is believed to have increased her holding in Australian media group Fairfax to more than 15 per cent, reports said Friday.

In a move set to bolster her bid for a board seat, the mining billionaire has boosted her position in the firm which has major newspaper and radio holdings by two per cent, Fairfax newspapers said.

Fairfax's flagship masthead The Sydney Morning Herald said Rinehart was thought to have hired brokers to eventually take her stake to 19.99 per cent which it said was the maximum allowable before she has to make a takeover offer.

Rinehart, who is worth Aus$29.17 billion (S$37.29 billion) according to an annual index by Business Review Weekly, has so far been unable to secure a seat on the Fairfax board despite being the company's biggest shareholder.

She has been highly critical of the media group, last month describing the company as "ailing" and urging the chairman to do more "rather than merely hoping for improvements in circulation, revenue and share price".

Rinehart's initial move into the company earlier this year prompted the government to flag stronger media ownership laws amid concerns that the mining mogul, who has been critical of the government, wanted to build her influence.

But some fellow stakeholders reportedly support giving her more sway, including Sydney radio network owner John Singleton who said the board's refusal to hand her a seat was "the worst corporate insult I've ever seen".

Fairfax, which has newspaper, radio and digital interests and is the main rival in Australia to Rupert Murdoch's News Limited, has been struggling with falling print advertising revenues.

It recently moved some 66 production jobs offshore, prompting a snap 36-hour strike.

Shares in the company have fallen nearly 85 per cent in the past five years. It was trading flat Friday at 60 Australian cents.