True Aussies use it when they're generally suprised or shocked.

An Australian exclamation of surprise or bewilderment. Interchangeable with words such as "Blimey" or "Struth". Made famous by our champion crocodile hugger, Steve Erwin.

EX Webster:

Main Entry: cri·key
Pronunciation: 'krI-kE
Variant(s): or crick·ey /'kri-kE/
Function: interjection
Etymology: euphemism for Christ
Date: 1838, England
-- used as a mild oath

An old English public school exclamation of surprise as exemplified by Frank Richards' Billy Bunter stories. More recently strangely adopted by Australians.

An expression of surprise, current amongst English schoolboys at least as early as the 1930s.

Polite Australian abbreviation for "jesus, seriously, you gotta be fucking SHITTING me."

Australian slang used normally when someones amazed or astonished at something.