Was "sexting" an Australian slang term originally?

The OED entry would suggest that it originated in Australia. At least, that is the first example they have.

The action or practice of sending or exchanging sexually explicit or suggestive messages or images electronically, esp. using a mobile phone. Cf. sext n.2

2005 Daily Tel. (Austral.) 2 July 87/3 A telling aspect of his sexual farragos is the use of his mobile for sexting (texting).

2009 Tennessean (Electronic ed.) 4 Jan. [She] didn't come across ‘sexting’ until college.

2011 S. Petrow Compl. Gay & Lesbian Manners x. 265 People caught up in the sexting frenzy of late may well be chagrined in the future to find out how widely these photos are distributed.

2013 Wall St. Jrnl. (Electronic ed.) 24 Aug. a13 He regretted not publicly disclosing that his sexting went on for months after he resigned.

That is just the first instance of the word in a print publication. The first use of the word likely happened years before that.

I don’t think you have any evidence at this point for what country it started in. The Australian article reads like they are using a common term, not coining the phrase or celebrating its novelty.

The best bet is to try and get in touch with the author of the Australian article and ask them where they got the word.

In the March 4th 2005 movie Be Cool, during a dance scene, the Black Eyed Peas are preforming and there is line in their song with the words "texting, and sexting me".