Some stamps are printed on watermarked paper and identifying these provides insight into the years that particular stamp was produced. Values of a particular stamp also vary according to which watermark was used so it is wise to sort your stamps on a watermark basis. There are commercially available detectors to highlight the marks but the simple method is to place the stamp face down on a piece of black card - the mark should then be visible but reversed. Our club has an electronic watermark detector for use at meetings.
Watermarks used on Commonwealth of Australia Stamps
Single "Crown over A" type. Used on the Kangaroo & Map series and the King George V definitives.
Most commonly described as First, Second and Third types.
FIRST SECOND THIRD
The "Large Multiple Crown over A" was the next type used, predominately on King George V issue of 1918 to 1920
The "Small Multiple" started around 1926 on King George V issues of and ran to 1930
The "CofA' commenced in 1931 and was the final use of watermarked paper on Australian stamps. Its swan song was in the mid 1950's. Most stamps post 1955 were on plain paper.
Watermarks can be sideways or inverted referenced to the design on the front of the stamp and some people actively seek these out as legitimate varieties listed in specialist catalogues.
It is important to note that not all stamps issued in the later use of watermarked paper were actualy printed using it and some issues exist solely on unwatermarked paper and others in both forms. Always consult reliable reference material to determine the correct placement of a given issue and remember that perforation guage can also vary on stamps appearing to be identical in all other aspects.