Because australia signed ACTA is it already immediately in effect for us?

I think it because effective the instant it was signed. After that it's just a matter of getting the checks in place for Australian networks.

I don't know how it works down under but a treaty in most parts of the world is an agreement to pass local laws that uphold the stipulations of the treaty. So you may very well have to have your legislative bodies down there both ratify the treaty and write and pass laws that uphold its mandates.

I guess the question is:

is it worth the risk to back up my GBA games on the PC? or is there even a risk yet?

Signing is not enough, it also needs to be ratified. Take for example Poland, where ACTA was signed, but not ratified. I am not aware of any ratification gone through yet, and as fas as I know - it need at least 6 ratified countries before it can come into effect.

So then its kind of safe to back up my GBA Games?

The reason i ask this is that i was recently in a storm in which some of my GBA games got destroyed (Queensland Australia, but the whole east coast is getting flooded here.) and i want to back up the rest before anything else happens.

ACTA don't work till is ratifised by all I think...

Yes, just don't tell Karel De Gucht.

To quote myself from an earlier thread:

A treaty can never have "legal power" as it is signed by the executive branch of the government. Therefore treaties like ACTA need to be ratified by the legal body to become effective. Usually treaties also get "transformation laws", because they are often to vague to actually work directly as laws.

That was my point

It's in Article 40: 30 days after ratification, so you're right: any country that seeks to bind itself to ACTA must ratify by that country's customary way of ratifying treaties. For the early ratifiers, it takes effect 30 days after the first six countries have ratified. For later ratifiers, 30 days after that country ratifies. And it doesn't actually have any teeth until countries pass implementing legislation ("transformation laws").
Actually, without ratification, implementing legislation, and enforcement, it's only a great many pieces of paper :)

Perhaps use VPN/SSH to a cheap foreign server to do it? Use whole disk encryption at home (Truecrypt works fine) and memorize a 20 character or more password.