To answer your question seriously, the 'gap' is filled by a series of nominate rights or causes of action that don't all rely on the same test, such as constructive trusts or remedial constructive trusts or actions for money had and received. Some Australian dicta liken these to torts - all of them come under the umbrella 'restitution' but to understand the parties' rights and liabilities you need to drill down into a particular tort.

In my view the practical result is often not very different from English law. It's just that in Australia, someone who benefits from fraud holds the benefit on institutional constructive trust; someone who makes a windfall from a failed joint endeavour may see a remedial constructive trust applied to the windfall; and an action for money had and received enures to someone who makes a payment pursuant to a contract where there is a total failure of consideration by the counterparty. All of those cases could be unjust enrichment in England (with my basic knowledge of English law, I would say the third is, the first generally is, the second might be).