austtr, you cannot repossess it if they change the info at the registrar, removing you as tech contact, and change the password at the web host so you can't access by FTP to kill the files. If they're unscrupulous, they could conceivably do this if they suspect that you might pull the site.
Before anything further, I would make sure of having backups of of everything, including all documentation - take screenshots of the connection via FTP and/or control panel, verifying dates, filenames, etc., any and all things to substantiate your work, including screenshots of major site pages.
Let's question ownership. In your example with the fridge, they can repossess it because it is still their property - not your property until it's paid for. When you pay for it, it becomes your property.
By what criteria is that website that client's property?
I had a similar conflict a little over year ago, on a small scale, but had to check a lot of similar issues out, which I did, in depth.
Without going into details, the client paid the required 50% in advance, sent the check by Airborne with the signed contract, and then proceeded to try to up the number of products from 55 to 148 without paying a dime more. No dice. I learned invaluable lessons from that hassle on what could potentially happen.
The web site was still MINE at this point in time - and I yanked that site off the server faster than you can say hot coffee.
An important question is: Who owns the copyright to a web site? Don't know about Australia, but here, the entire site except those elements that are provided by the client -all the graphics, HTML, design elements, the whole kit and kaboodle is the intellectual property of the creator - ME in that case. In your case, you. Copyright and all attendant privileges stay with the creator unless and until they are expressly, in a legal fashion in writing, turned over to someone else, relinquishing any further rights. Our intellectual property cannot be used or distributed in any form without our permission.
What I learned form a chat friend who is a consultant for government contracts is that, written or otherwise, a contract (agreement) always consists of two elements - offer and acceptance.
In my case, offer and acceptance would have been valid even without the contract - reinforced by the description on her check, and doubly reinforced by the fact that I had copies of correspondence concerning her already having already seen and approving the site prior to sending the check. I actually did most of the work in advance (with documentation).
In your case, obviously an offer was made, which they most certainly did accept, since they are paying for hosting to house the site you created, which IMHO, is still your property. Remember - just my opinion.
These things vary not only from country to country, but within the US, other than copyright laws, which are wider in scope, from state to state.
Here's an American site with some good advice and information:
Here's an Australian copyright site I found at Anzwers:
You'll get a chuckle at this line at the bottom of their index page: If you are an indexing robot, click here. :)
I'm sure someone much more knowledgeable than myself will give better information, but I did want to share some of my experience with a similar issue and bring up a few pertinent points.
Disclaimer: as is always the case on the internet, all statements are no more than the opinions of the person stating them (in this case, mine) and should not be taken as advice, in any way authoritative, or even assumed to be correct. They are merely opinions, and people should, for their own benefit and protection, always consult with an accountant, attorney, or other qualified professional or professional organization or government agency in the locale where they live.