According to the Windows blog, Windows 8 will be available from 26 October.
The last official piece of information on availability was simply stated to be sometime in late October, with a release to manufacturing (RTM) in August. Windows 8 will be available either on a new PC or through an upgrade from 26 October.
According to CNET Australia's sister site ZDNet, it's likely that this will also be the date when Windows RT-based Surface PCs and tablets will be made available as well.
Microsoft has not officially said that Australia will be one of the 131 markets due to receive the upgrade for US$39.99, it seems likely, given we qualified for the previous US$14.99 upgrade for PC buyers.