Communications Minister Stephen Conroy says mining magnate Gina Rinehart has no right to trash the reputation of Fairfax Media by overriding the company’s charter of editorial independence.

In an unusual display of bipartisan politics, shadow minister Malcolm Turnbull has also voiced his concerns about a takeover by Mrs Rinehart, and warned that “if Fairfax … were seen to be a mouth piece of Gina Rinehart and a spokesvehicle for the mining industry, that would undermine its business model dramatically”.

The coal and iron ore billionaire recently increased her shareholding in the publishing house to 18.67% and is poised to seize as many as three seats on the board. She has intimated that she wants the power to hire and fire editors at the company’s iconic mastheads, including The Sydney Morning Herald and The Age.

For months the Fairfax board, led by chairman Roger Corbett, has resisted pressure by Mrs Rinehart for representation, largely over concerns about her intention to influence editorial direction. But the board is expected to capitulate soon as the world’s wealthiest woman increases her stake to almost 20%.

Senator Conroy told ABC Radio National that Mrs Rinehart was “entitled to representation but what she’s not entitled to do is trash the brand for all the other shareholders. She should be aware that that Charter is something that the readership of The Age, the Sydney Morning Herald believe in and have supported over many, many years. And if she was to directly interfere and breach that Charter, it would actually lead to a crisis of confidence in the – among the readership and if the readership deserted, then the share price for every shareholder would decline.”

Several reports have claimed that Mrs Rinehart believes directors should be able to overturn the charter, if it is in the commercial interest of the organisation. On a trip to Australia last year that was reportedly funded by Mrs Rinehart, British climate change sceptic Lord Christopher Monckton decried the state of Australian media and said that “devoting some time and effort to encouraging those we know who are super rich to invest in perhaps even establishing a new satellite TV channel – it’s not an expensive thing and then get a few Jo Novas and Andrew Bolts to go on and do the commentating every day but keep the news straight, fair and balanced, as they do on Fox, that would break through and give to Australia, as it has for America, a proper dose of free market thinking”.

Senator Conroy said it was the sort of thing that “sends shudders down the spine of many readers of the Fairfax publications and Ms Rinehart needs to be conscious that the 80% of shareholders that she doesn’t directly represent would take a view that anything that diminishes the brand – anything that diminishes the support for the newspaper among readers will destroy value. It will destroy their value.”

Mr Turnbull agreed that the reputation and credibility of Australia’s oldest newspaper businesses could be at threat from vested interests. “There’s a subtle difference here between the very wealthy individual like a Rupert Murdoch or a Kerry Packer in days past, or you know the Fairfax family in days past who were very wealthy and had strong opinions, exercising those opinions through their publications, and somebody who is also very wealthy but has very substantial vested interests in other parts of the economy.

“Someone with strong vested interests in other parts of the economy, there’s always going to be the question, well is the newspaper opposing this Government policy in order to benefit the vested interests of substantial financial investments of the owner. So whether Gina is on the board with 20% or buys the company, if she wants those newspapers to maintain their influence then they have to be seen to be objective and independent and not pushing a barrow that happens to be coincidental with the financial interests of the owner.”