Philanthropist Dame Elisabeth, mother of media magnate Rupert Murdoch, died aged 103 in 2012. She left the historically significant Cruden Farm in Langwarrin - then valued at almost $10 million - in the hands of daughters Janet Calvert-Jones and Anne Kantor, granddaughter Judy Paterson and accountant Ian Evans.

These trustees now propose to carve off almost a third of the property in Melbourne's south-east and rezone it for sale as 116 generously sized housing lots.

The land is zoned for rural conservation, which does not allow large-scale housing development.

Only Planning Minister Richard Wynne can rezone the land.

The principal purpose of the Keith and Elisabeth Murdoch Trust is "the retention of Cruden Farm as an area for public recreation".

Dame Elisabeth in 2007 said it was her "wish that the gardens and other areas be retained largely in the form in which they are at present".

According to plans lodged with Frankston Council in April, 14.6 hectares have been identified as "surplus to the main estate" and should be sold to support the farm's viability and to provide "increased public access".

The plans were released under freedom-of-information laws to local environment group Defenders of the South East Green Wedge. Secretary Barry Ross said the subdivision should not be allowed.

Converting the "surplus" land from a rural conservation zone to land for housing would set an "appalling precedent", he said.

"If this gets through, there will be a whole bunch of other landholders saying, 'We want some of this too.' There's a lot of land-bankers that own land in the green wedge and they are just waiting and waiting and lobbying and lobbying."

Karen Stewart lived directly behind Dame Elisabeth.

"She told me on a couple of occasions that it [Cruden Farm] would always remain as it was when she passed away," Miss Stewart said.

"If they take away the big chunks of land, making it into a housing estate, it's going to be horrible, it's not going to be Cruden Farm," she said. "What's stopping them taking more of it away in the future?"

But trustee Ian Evans said what Dame Elisabeth had wanted was being carried out.

"The trustees are pursuing Dame Elisabeth's wishes and vision for Cruden Farm."

Mr Evans said the land sale would provide funds to allow the estate to be maintained and opened to the public in perpetuity.

He said the trust's application for redevelopment had been deferred but would likely be resubmitted by the end of the year.

Elisabeth Murdoch was given Cruden Farm as a wedding gift in 1928 when, aged 19, she married newspaper executive Keith Murdoch.

While Dame Elisabeth's will valued Cruden Farm at $9.82 million, an estimate in April by real estate agency Savills valued the 14.6 hectares the trust wants rezoned at between $25 million and $30 million.

Dame Elisabeth's will states that trustees are able to "charge an entry fee" to Cruden Farm, to "construct facilities for the use of the public", and to "do any act or thing which in the opinion of my trustees is necessary for the proper and efficient management of the trust fund".

Suzie Webster, from Friends of Langwarrin Outdoors and Waterways, said the group planned to take its Save Cruden Farm petition with almost 5500 signatures to Planning Minister Richard Wynne.

Mr Wynne's spokesman said the government was aware of "proposals to make Cruden Farm more accessible to the public, but our position on the urban growth boundary has not changed."