Doug Lombardi has just spoken to Shacknews:

"We were surprised to hear of this news yesterday," the executive informed us. "Obviously, everyone at Valve is pretty bummed."

"It would be a shame if folks in Australia, or anywhere else, are unable to purchase Left 4 Dead 2 because of a ratings issue," Lombardi lamented.

Left 4 Dead 2 is available everywhere else on the planet on November 17th.  Meanwhile feel free to download any R18+ mod available whilst the Australian Office of Film and Literature Classification board targets only what gets deposited on their god damn laps.