Sorry guys that I've been really strapped for time. It's 1am, should be in bed

The stamp is a Woodblock printed by S Calvert - first issued imperf in 1854. The group of stamps known as woodblocks were hand engraved in boxwood and surface printed.
The imperf, roulette and perfed varieties are all from the same printing. Calvert printed 1 million stamps in a dull bluish green shade on tinted yellow paper. (The yellow tint was actually provided by an initial lithographic printing.) The stamp was issued on 1 Sep, 1854, imperf.
Rouletted copies are all from the Melb GPO window clerks, from Aug 1857 - they usually only rouletted one or two sides. There was no other rouletting of this stamp.
In April 1859, the entire remaining stock of Calvert's printing was perforated by F. Robinson. The line perforating machine was gauged at 12 - however it's actually usually a little lower - anywhere from 11˝ to 12, but rarely over 12. It was a single line cutter - a single row of perforating pins. So perforations will vary between these numbers.
The stamp was in use until Nov 1864. So the supply lasted a long time. Not only that, but in the end only a little over 300,000 had been issued in total (including imperf, roulette and perfed.) The remaining stock was destroyed.

So the stamp is SG 82. It's most likely as Rod said, cut on 2 sides with scissors. The reason for the cutting is probably not determinable. Interestingly, there is one known used copy from 1863 imperf at right, but it's a much bigger margin. The stamp was printed using a single plate of 25 impressions - however the contract was for sheets of 50, so 2 impressions were used side by side. There's a wider gutter between the 2 impressions, so stamps along the gutter will have wider left or right margins. The stamp has been plated (but not by me!

)
The Melbourne VICTORIA duplex was in use for the last few years of this stamps life. So it would appear to have been used some time 1861 - 1864.
The 1864 issue (SG 129) mentioned by Rod & Tony was a different printing and appears blue, not green. It was on greenish paper - yours has the yellow tint of the Calvert printing.
Jeff - unlike the US, Australia, including the early issues by the colonies, perforate all the way around the sheet, including the outside edge. So you don't get straight edge stamps like you do with US issues. Usually anyway.

P.S. Did you notice the words "one" & "florin" in the scroll work beneath Victoria? One florin was 2 shillings. The words were to make forgery more detectable.
Hmmm, that was a bit longer than my intended "it's too late for a detailed post"

Balf