On the off chance that someone may have had the same issue ... I just tried to preorder The Witcher 3 a couple of times and it was declined. I then bought Loom with the exact same credit card details and it was fine. I then tried The Witcher 3 again but it was declined. Is there something different about preorders? Back when I preordered The Witcher 2 I didn't have any issues.

Normally there should be no problems, best option for you would probably be to contact the support.
(Might take a bit for them to answer since support is a bit slower on weekends)

http://www.gog.com/support/contact

if you pay through paypal, then it can be that paypal is often bitchy. for example:

paypal decides that you allowed to buy stuff if it costs less then 20$ for the next month (you won't get informed about that). then you won't be able to buy witcher 3 through paypal, while you can buy 6$ games through it.

You may want to contact your credit card company. At times (and in some cases reasons) limits are put on spending. There are certain flags like amount of charges, frequency of charges, where the card was charged, etc. and they credit card companies have fraud detection systems that go into overdrive. While it can be annoying at times, it's really a good thing since it slows down damage that can be caused by anyone who steals your card, be thankful they have it in place!

Call the number on the back of your card, speak to a rep and let them know which merchant you're dealing with and how much you'd like to allow yourself (have some reserve, don't go overboard) on a transaction.

They may be able to accommodate your request and fix it so that you can make the purchase.

Hrm, maybe The Witcher 3 is banned in Australia for having excessive nudity and sex with monsters or something?

I don't think any funnel web spiders, inland taipans, crocodiles, lionfish or centipedes were harmed during production.

Thanks for the replies. Well, the on topic ones, anyway. I was too lazy to follow this up during the week and tried purchasing it again today. The first attempt failed but in a different way. The page just got stuck with the spinning button. The second attempt worked, though. I did enter my name a bit differently this time, though, including my middle initial. I've never had to before. Maybe the checks are more stringent now. Like JDelekto said, this is probably a good thing anyway to prevent fraud.