Firstly - congratulations!
As the others have said you will be fine to have an appointment when you get back. The only thing I would perhaps look into is if you want to see a specific ob or get into a birth centre of midwifery practice you might want to see if you can set up a booking now and get a referral for that service when you get back into Australia. Not all services will accept a booking without the referal but some do (I booked with my ob then got the referral from my GP before my first ob appointment).

On the husky sled ride - I've done one (but not while I was pregnant) and I didn't experience anything that I would have concerned me if I was pregnant. The sled I went on was a 4 person one, where you sit on sled (plenty wide enough for my butt, so worries there) and sit your feet either side in kind of a squatting position. It was probably like a mid pace motor bike ride speed wise, but it felt safer than a motorbike because what we were sitting on was a lot bigger and steadier and we weren't doing sharp turns or anything like that, and at no point did I feel like I was going to fall off or anything.
I wouldn't like to say, 'yeah you will be fine go for it' because only you can assess if you are ok with it. When I went (in Sweden) there were husky tours every day so I had seen the dogs out and about before I went and saw what it was like. Perhaps you can check them out before your ride is booked and see what you think? I reackon when you see it you will be fine with it but I'll leave that to you.
Also, enjoy it! It was so much fun and the dogs are great, a little highly strung but fun all the same. It is definately a once in a life time experience.

HTH