The 2d blue 3rd Sideface issue (SG 234), was issued from April 1897 to September 1906. There were several plates during the life of this issue. Plate 3 was introduced in about September 1900 and included the "cracked head". In June 1906 plate 3 was rearranged following the repair and replacement of the "cracked head". Known as 3a, this plate had a very short life - June to September 1906.
Very shortly after this rearranged plate was introduced, some very badly blotched stamps were printed. They as now known as the "Diseased Queen" or the "Blotchy Queen". It is probable that they are from the first printing of plate 3a as the only known dated used copies are dated 18 June 1906.
Samuel Dalby, writing in the Australian Philatelist, 7 June 1916, p. 159 (Queensland: The 1897 Series of Electrotypes - II), describes what happened:
"Also a few sheets presented a curious phenomenon: the head on stamps in column 1, on lines 2 to 6 (Nos. 11, 21, 31, 41, 51) had an unsightly mottled appearance, blotches of blue marring the shading lines from the diadem down to the neck. An official explanation was that through carelessness, part of the ink was insufficiently mixed or thinned before being served to the inking rollers; some of the thicker portion having got down to the end of the roller the colour was applied too solidly near the edge of the plate, and clogged the shading lines during several feedings of the press".
It is not listed as a variety in the Stanley Gibbons catalogue. It is however listed in Brusden White, The Australian Commonwealth Specialists' Catalogue: Kangaroos and the Early Federal Period, 1901-12 as Q16c - Blotchy Printing and priced at $200 mint and $125 used. An included note explains: "The blotchy printing is very scarce and occurs for a short period in September 1906. It varies from stamp to stamp and at its most extreme can almost totally obliterate the Queen's head. It is believed to be due to an unsatisfactory mix of ink." It would appear from this description that the author had not seen the block dated June 1906 as it almost certainly was not still occurring three months later. As soon as the error was detected, the printing would have been stopped and and any existing diseased Queen stocks destroyed.
This is a composite illustration shown in Kenneth Scudder, Queensland Postage Stamps 1879 to 1912, p. 266A block of 6 postmarked Crows Nest 18 June 1906. Positions 01, 11, 21. From the collection of Dave Elsmore