The following firearms were used in the film Australia:
Winchester Model 1895
Drover (Hugh Jackman) carries a Winchester 1895 lever action rifle in the carbine configuration. Since Australia is a member of the British Empire/Commonwealth and allied with Great Britain it's probably a safe bet that the carbine is in .303 British. Probably the most popular sporting caliber in many of the British Commonwealth nations for many decades.
After speaking with the film's armourer (John Bowring), I can confirm that both Winchester Model 1895 carbines were chambered in .303 British. [Peter Collins]
Single Action Army
Alexander Henry 450 3" Double rifle
Neil Fletcher (David Wenham) uses the butt of his Double rifle to knock Lesley "King" Carney into the crocodile infested swamp waters.
Smith & Wesson Model 10
An Australian policeman draws his Smith & Wesson Model 10 and shoots a feral dog.
Lee-Enfield No.1 Mk. III*
Several Australian soldiers and various other characters use Lee-Enfield No.1 MkIII* rifles.
An Australian soldier is briefly seen carrying a Lewis gun.
Nambu Type 14 Pistol
A Nambu Type 14 Pistol is used by a Japanese officer.
Arisaka Type 38
Japanese soldiers are armed with Arisaka Type 38s on Mission Island.
A disheveled Anti-Aircraft Gun can be seen after the bombing of Darwin.