I've stayed in both chains before, but not actually in Melbourner, but the two key 'budget' hotel groups in Australia are best Western and Choice (Formerly Flag Inns). There is a Collins Street 'Quality Hotel, that is central to everything: I've been in the reception area, but never in one of the rooms...it's just down from the Hyatt. (go have a look...the atrium is lovely). The Quality is within your price range.
http://www.choicehotels.com.au/hotels/home
http://www.bestwestern.com.au/Default.aspx
Enjoy Downunder Mate!
Fr Bill