"It's nearly half a metre high by half a metre wide and very seldom do paintings of that substance and scale become available on the open market," Plumbly said.
"They are in the vast majority held by public galleries and institutions which means they obviously never come up for sale.
"The ones that come available on the market are generally more like 20cm by 15cm, so quite diminutive."
Also "particularly special" about Goldie's Paparangi portrait was its "provenance" - the documented evidence of an artwork's ownership and location over time.
"It was purchased by the current owner's father who bought it from Goldie's Auckland studio literally, and then it's been passed by descent," Plumbly said.
"Pretty amazing to have a painting that's the best part of 80 years old that's never been made available to the market before. I don't think it's ever been exhibited."
Reliable provenance was especially important for Goldie's work, because renowned Kiwi art forger Karl Sim had especially targeted Goldie's paintings, and was convicted for forgery in 1985.
Goldie's paintings' have grown enormously in price, from about $40,000 (converted to 2017 values) during his lifetime.
In 2016, Goldie's A Noble Relic of a Noble Race portrait sold for $1,339,500 - the second most expensive Kiwi artwork ever.
Art critic T. J. McNamara wrote at the time of the auction that Goldie was "easily the best-known and most-admired painter in the history of art in New Zealand".
The Takahi Atama Paparangi, A Rangatira of high lineage painting up for sale on April 4 is estimated to sell for $750,000 to $1,000,000.
