Yes, this can be done through your 'My Events Portal'. A link to your portal can be found at the bottom of your ticket confirmation email. Once logged in, click on the option to 'purchase merchandise'. From here you can add spectator tickets and other merchandise items such as parking, thongs, trucker hats, socks, drink bottles and stubbie holders.
Spectator tickets are also available as stand alone tickets in the weeks prior to the event.