In no particular order:
--Catherine Jinks. Evil Genius is one of my favourite books--I have a soft spot for evil child prodigies, and Cadel such an amazing one (in fact, he's #1 on on my Top Ten (Or So): Evil Child Geniuses list); its sequel Genius Squad contains a character who made it to my Top Ten (Or So) Fictional Canadians; her Pagan books contain one of the awesomest Bromances; her Reformed Vampire and Abused Werewolf books are funny and fresh takes on characters that have become far too cliched recently.
--Garth Nix. Completely awesome and original--I haven't come across someone quite like him before, and this is said among a list of very original authors. Read Sabriel and it's two sequels, Lirael and Abhorsen (this is a command, not a request). Also check out his short stories--he's written lots that are spread out over all sorts of fantasy anthologies.
--Jacyln Moriarty. The Ashbury/Brookfield series is so cool that I can guarantee any one to be a fast and enjoyable read with great characters; see The Year of the Secret Assignments, The Ghosts of Ashbury High, and especially The Murder of Bindy Mackenzie--my favourite!
--Justine Larbalestier. Her Magic and Madness series is quite interesting, with complex characters and a fascinating magic system. Liar and How to Ditch Your Fairy are also pretty good.
--Alison Goodman's Singing the Dogstar Blues. Cool everything: world, characters, language, plot.
--Elsbeth Edgar's The Visconti House. Great Middle Grade fiction. Not my favourite ever, but still great and worth reading. Plus it's Australian.
--Andrea K. Höst, of the Touchstone trilogy, the Medair duology and more. Technically born in Sweden, but raised and living in Australia. Her books are scifi and fantasy with twists and surprises and survival and romance and complex, interesting worlds.
Honourable Mentions: Juliet Marillier (of Wildwood Dancing and Daughter of the Forest) and Karen Healey (of Guardian of the Dead) are both New Zealanders living in Austrualia, so I couldn't properly add them.