I've never heard it in the UK, or from elsewhere.
Here's are three examples of how it's used, from Superlinguo:
Goddam. The one day I am not writing on question time, the PM gives everyone an early mark. Why is life so immeasurably cruel and unjust?
The net is down at work, but it doesn’t mean we can stop working or get an early mark… #fml #wtf
Early mark for coffee today. I heard the inner call at 2.40pm
Searching for the phrase on Twitter Map ("See and visualize twitter users tweets overlayed on a map") only gives three positive results, all from Australia:
120dollarsfood from: Melbourne
@iamevilcupcake Early mark.
Mel452 from: Sydney, Australia
Woot early mark. It's so dead at work. Going crazee!
LyndonKeane from: St George, Queensland
Due to a general lack of interest, I've decided to give myself an early mark.
Looking at early usage, an early mark was something that was earned in return for good results. Here's the October 2012 draft entry from the Australian National Dictionary Centre.
Here's some interdatings. First, from "From Our Mail Bag" in the Worker (Brisbane, Qld.), 26 May 1952:
Dear Uncle Toby,
I am sending a coloured picture for
Ihe competition. I had a test on
Friday and received 90 per cent, in
dictation and 87 1/2 per cent, in sums.
I also received an "'early" mark. It is
raining down here. I have a poisoned
foot. — Your unseen niece.
And in "Football Briefs" of the Barrier Miner (Broken Hill, NSW), 5 May 1954:
forward Cec Fields has
some fans already. Saw
one little chap offer him a
chewing gum as he left the
field at half time. Warrant
an early mark, Cec?