And some further musings...some based on common law which we may share, others just the ramblings of a tired legal mind, which ramblings may or may not have relevance. And note: copyright law is not my strong point.
(BUT FIRST A CAVEAT: I pose some questions to think about here. DO NOT ANSWER THEM IN THE FORUM. If you are thinking about litigation you don't want to put your facts out in public. Even with a pseudonym, the other party may figure out it's you and a public discussion could undermine your case.)
1) Consideration: The contract is valid and not an empty one, generally, if you can tell who the parties are, what the subject of the contract is, the essential terms, and you have consideration: that is, a quid pro quo. Their quid is your work on the website. Your pro quo is their promising to use the hyperlink.
2) Ambiguities: Just who drafted the contract may be important if there are ambiguities, and it sounds like there probably are.
If you both worked on drafting it, then the court/tribunal may just try to see what clues are within the document to show what was the intent of the parties. If only one of the parties drafted the document, then it may be that in Australia, as here, the ambiguity will be construed against the drafter. (It's like when moms have one child cut the treat in "half" but the other child gets to choose the "bigger half".)
3) Things which could be important (particularly if there would be a question as to whether the matter would be treated purely as a contract matter or whether copyright law might govern what could or could not be contracted) might include:
-any indication in the contract that you were keeping your copyright in what you created and only granting him permission (i.e. licensing him) to use your creations so long as he provided the prescribed hyperlink;
-what sort of attribution the contract required through the hyperlink: was a particular form specified and did that form include a reference to copyright?
-what terms were used to describe the web-page--was it treated as a single creative unit? did it vaguely reference your "work"? What might a third party think was covered?
-What sort of information is on the hyperlinked page?
-did the contract make any reference to what the remedy would be in the event of breach?
My guess is that
if Australian copyright law were to be that the creator of a work retains copyright unless he/she affirmatively and specifically transfers that right, then 1) if his alterations would be considered "derivative" works--requiring your permission or 2) even if his revisions weren't derivative, if the separate unaltered parts of your website would remain as under your copyright, in those cases you might well have a copyright case against the website owner if the only legal way he could display your work was under the "license" you granted by the contract -- and you can show that contract to be materially breached by his failure to continue to show the hyperlink.
Hmmm. Here's another thought:
It was free but in a contract we both signed, he agreed to my right to promote my work and to seek attribution for the work derived by me.
The contract stated that attribution would be in the form of a hyperlink in the footer of the homepage.
Words are important in contract disputes. You say he agreed to your "right to promote" your work and "to seek attribution" for the work... If that is the language in the document, that could be a killer. Saying you have a right to promote your work is different from saying he has the duty to promote your work on the web-page. Similarly your "right...to seek attribution" could be empty words--what you want is to have the right to
have attribution on the web-page (and him the concomitant duty to provide the attribution), not merely your right to "seek" it. Also, at least in the US, you wouldn't want to use the word "derived" as you have above. "Derive" has a specific meaning in the context of copyright ("derivative works") and so "created by me" would be preferable.
Anyway, it's past bedtime here so I'll leave you with those thoughts.... (The tireder I get the more I ramble...)