Notwithstanding the Stanley Gibbons introductory note that is often dismissively quoted when an enquiry is made of watermark varieties among the Australian colonial issues, the fact is that they are rare on some issues and common on others.
As the note states, on some issues the watermark varieties were "restricted to odd sheets".
It naturally follows that the value would be much higher for such stamps but "due to the circumstances" (i.e. lack of reliable/accurate statistics regarding the numbers printed in various orientations) they find that "..it is impossible to provide adequate prices..".
Nobody, for instance, should be conned into parting with an inverted crown over NSW watermark on a Queen Victoria halfpenny for the ordinary catalogue price by means of a supercilious reminder of that quote. The same holds for a 1d shield with inverted crown over single-lined A and many others.
On many issues, for the reason mentioned by 64idgaf or due to tete-beche plates etc., watermark inverted is easily found. Nobody, for example, should pay a premium for an inverted crown over NSW watermark on a 2d emu stamp.
The 2004 Australian Commonwealth Specialists' Catalogue provides information and relative pricing for issues from 1901-12 and other esteemed publications such as Hutson's 'The Stamps of New South Wales' and the works of Basset Hull give further insight but there is still much to be documented.
In the case of the 5d large diadem stamp shown by dcaraz1949, watermark inverted appears readily found for crown over NSW type II but for the 1903-1912 type III (1mm spacing between crown and NSW) the ACSC assigns a higher catalogue value for inverted watermark.
A comprehensive documentation of the known quantities of watermark orientation from the archival records, published works and a survey of the most comprehensive existing collections of Australian colonial stamps would be a grand work indeed.
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