Supreme Court Justice Kate McMillan found Don Baxter was entitled to the $350,000 payout, instead of the original $50,000.

Justice McMillan said the original payment to Mr Baxter, who was diagnosed with cancer in 2013, was inadequate.

Mr Baxter, the former executive director of the Australian Federation of AIDS Organisations, who was awarded an Order of Australia Medal this year for his services to the community in the prevention of HIV, believed he deserved more from his father's estate and took his case to the Supreme Court.

Mr Baxter, 64, who is retired and lives in a million-dollar two-storey terrace house in Darlinghurst in inner Sydney with his partner Rob Holland, sued his brothers, Bill, 67, former minister for roads and ports in the Kennett Government, and Stewart, 61, who were the executors of their father Allen's estate. Allen Baxter died in January 2011 aged 96.

Justice McMillan said Don Baxter's financial position and health had deteriorated since his father had passed away. "Although the defendants were critical of Donald's decision to step down from his paid work, it was inevitable with his being in his early 60s that he would not work for much longer," Justice McMillan said in a judgment handed down on Friday.

"He is now dependent on his superannuation for his income, which will be exhausted in 25 years. "At present, his income more or less matches his outgoings. He owns, in effect, half a house with his partner.

"In my view, Donald's current financial situation is such that he has no financial buffer against future financial contingencies.

"The authorities show that concepts of proper maintenance and support encompass enough to keep the 'wolf from the door' and sufficient to 'keep the wolf from pattering around the house'; not just sufficient for the bread and butter of life but for a little of the cheese or jam that a just and wise parent would appreciate should be provided if circumstances permit."

Justice McMillan said it was agreed Allen Baxter's estate had been worth $742,000 when the case went to trial because most of his wealth "was transferred during his lifetime to the Baxter Family Trust and the Baxter Family Property Trust".

"This is not untoward and no criticism is made of that fact. The result, however, is that the quantum of the estate is reduced."

The judge said that around $6 million of the family's wealth was in either the Baxter Family Trust or the Baxter Family Property Trust, controlled by either Bill Baxter alone or Bill and Stewart together.

Bill Baxter receives a lifetime, tax-free annual parliamentary pension of $145,000.