Odd use of the word 'rare'. Each painting is unique and is thus exactly as rare as any other painting. Or does it refer to the artist's total number of works?
The artist concerned here "is considered as one of New Zealand's leading artists" and has a "unique painting style", and so there are very few paintings that are like his, and (probably crucially, although I'm not familiar with the specifics) plays a role in New Zealand national identity.
You can get plenty of expressive landscape paintings, in the sort of style of an unimaginative Van Gogh, produced by hundreds of artists throughout the 20th century, and most are perfectly pleasurable to look at, but you've never heard of the artists, and the works are easily available for hundreds to a few thousand dollars -- because they are not rare, it doesn't particular matter which of the many available paintings you end up buying, as long as you enjoy looking at it.
For example, if an artist had most of their work destroyed, any remaining paintings would be a rare painting by that artist.
Or if a historically significant figure had only been painted by a small number of people, any painting of that person could be described as rare.
It's probably fake anyway!
E.g. Edvard Munch's "The Scream" [1] exists in four different versions that are all referred to by that name (as well as 45 litographs).
Other times simply because the artist painted the same subjects a lot.
Though by my own definition, it means the scribbles on the side of my notepad are equally (if not more) rare. So disregard my comment as 5.30pm without enough caffeine nonsense.
Or even just "A Rare example of a great masters work." which seems perfectly reasonable to me.
He probably just means valuable or unique(as a kind of painting whether by authorship or other kind). Title in a previous version might have been "Rare Gottfried Lindauer painting is stolen...", but was shortened.
On a different note, how can you buy without confirming authenticity?