John Flynn, who is now retired and living in Darwin, told Awaye! he meant to sell the rights to an existing copyright agreement, which was valid until 1990, rather than for the full copyright period, which extends for 50 years after an artist's death.
"That would have been an extra 27 years, and if I did, it was a mistake on my part," he said.
"Or it could have been that I was on holidays and somebody else in the office signed it without realising it should have only been for seven years.
"That bothers me, that I or someone in the office may have signed something which wasn't a reflection of what we were doing. Unless I see the file and saw something different on the file ... it was never my intention."
An actuary valued the seven years of copyright at $8,500 in 1983. Mr Flynn did not consult an art expert regarding the valuation.
"In hindsight, a cautious public trustee would have done that," he said.
Copyright owned by North Sydney family
In 1957, two years before he died, Namatjira agreed to a partial and limited assignment of his copyright to art dealer John Brackenreg's publishing company Legend Press, in exchange for a 12.5 per cent royalty.
Mr Brackenreg and Legend purchased the full copyright in 1983, and the Brackenreg family, who own an art gallery in the northern Sydney suburb of Artarmon, will retain their hold over Namatjira's copyright until 2029.
But Namatjira left a will in which he passed his assets — including the copyright — to his wife and children.
And according to a source close to the Namatjira family, Namatjira's widow Rubina, who died in 1974, also left a will in the form of a deed of family arrangement, in which she named several beneficiaries.
Colin Golvan, a barrister specialising in intellectual property law and a trustee of the Namatjira Legacy Trust, is highly critical of the sale and says Namatjira's estate was significantly undervalued.
"The public trustee should never have, in my view, parted with the copyright in the way that was done," he told Awaye!.
"It's indicative of the kind of state of incredible disadvantage that the copyright in Albert Namatjira could have been sold for such a small amount of money.
"You can imagine that return over the years would have very, very significantly exceeded the capital value which was recognised by the Public Trustee.
"It's another tragedy to add to the litany of tragedies that surround Albert's life."
Rights could be returned to family and extended
According to Mr Golvan, if Namatjira's copyright were returned to his family, there's an argument that it could be extended in perpetuity, as in the case of the published work Peter Pan, by the Scottish author JM Barrie.
In that case, the proceeds from the licensing of Barrie's copyright over Peter Pan flow not to his heirs, but to a children's hospital in London.
"We can and must reconcile ourselves to a situation which by and large needs redress," Mr Golvan said.
"And if we can't get redress in the case of the great Albert Namatjira, then where to do we start?"
Many of Namatjira's descendants live in poverty, with the sale of their artwork — in their grandfather's distinctive Hermannsburg style — their only source of income.
Namatjira's granddaughter Gloria Pannka firmly believes that he would have wanted his descendants to benefit from his copyright estate.
"It's the future we are looking at," she said.
"If we do get the copyright back, it would help us a lot."