Congratulations & welcome to I-Do
How far out of Brisbane are you prepared to travel?
I can think off the top of my head for "country venues", places like -
Dayboro - some wineries out there
Glass House Mountains
Or there are some "country clubs" down near the Redlands, or even places like Kooralbyn.
Alternatively, you could have something at Roma Street Parklands (for example) and decorate or theme it so it's a little more rustic ...
Looking forward to hearing of your plans