@FNM We have the Sierra Designs Flash 3 (regular model not the featherlight), and got it on Amazon USA.
You could see if Backcountry Edge will shop Internationally to Australia for you, as I see they have the tent for US $199 on Amazon, and they are a major stockist of Sierra Designs.
Campsaver have all 3 models except the Flash 3 regular. (They have the Flash 2, Flash 2 FL, and Flash 3 FL). See:
https://www.campsaver.com/s/sierra-designs-flashThey do definitely send to Australia, and in fact I believe they do free postage if you spend over US $400.
The Feather Light versions (which weigh less, using lower denier fabrics to save weight) have exterior pole structure with clips. The regular version have the pole sleeves and the ability to pull up the 2 sides for star gazing.
The reason I personally went with the Flash 3 regular was A) It was cheaper, B) I have a toddler so wanted the more durable tent, C) I liked the ability to pull back the sides for star gazing and extra ventilation during hot weather, D) We're using it mainly for short hikes in. We find it an immensely liveable pleasant tent to be in. It really enhances our camping experience. It's got great headroom throughout the tent, not just the centre. You can easily have 4 people playing cards in it, etc.
We are looking at potentially getting the Flash 2 FL for more hiking-based trips as well. And apparently we're not the first to have both the Flash 3 regular (for car camping / short hikes with camping) and Flash 2 FL (for lighter pack hiking trips). Although I am also looking at the Tarptent Double Rainbow for lighter hikes as it's only 1.2 kg for 2 person.
I agree with your brother regarding the Flash 3 being a colder less insulated tent in cold weather. It's fine in the rain - in fact it's GREAT in the rain, because you can have the option of watching the rain through the window, or zipping up the windows as much as you want if you were getting really sideways rain on one side. However if you are doing really cold weather regularly, I'd go for something more enclosed. (I'd probably go the Mont Moondance EX, or a Tarptent model like the Double Rainbow or Cloudburst 3.)
Have heard good things about the One Planet Goondie also, so I'm sure this would be good too.
If you're comparing, do note the waterhead rating and fabrics of the tents also. Not just the weight. (Unless you're doing big distances / up mountains and weight is really the major factor.)
Good luck - all the advice you've received here is good. I think you're going to end up with better tent than the ones you were initially looking at, regardless which of the options recommended by others you pick. They're all giving you good recommendations.