If you're a contractor who screws up like this, chalk it up to experience and move along.
Here we run into some very choppy waters in Australian jurisprudence.
While the USA has already published its third restatement of the law of unjust enrichment, and the courts of England and Wales have handed down a five-stage test for it, and despite being, along with Canada, one of the pioneers in the field of this area of the law, unjust enrichment in Australia is a mess with State Supreme Courts disagreeing with each other about what the High Court has said about it. This is not entirely their fault, as the High Court has said different things at different times, including an abrupt about-face around the turn of the century and a cautious peeking back over the shoulder in recent years.
This article (Kit Barker, 'Unjust Enrichment in Australia: What Is(n’t) It? Implications for Legal Reasoning and Practice' (2020) 43(3) Melbourne University Law Review 903) gives a thorough analysis of the state of the law, and the author's summary slides consolidate it nicely (Kit Barker, 'Unjust Enrichment in Australia A Commentary', (PowerPoint Presentation) https://law.uq.edu.au/files/46387/Unjust-Enrichment-Commentary-Kit-Barker.pdf).
First, unjust enrichment in Australia is not a cause of action, in the same way that "tort" is not a cause of action; it is an umbrella term that captures a number of causes that have restitution as a remedy. Maybe - both the Victorian Supreme Court and the High Court have questioned the idea of this umbrella, but the HCA has also embraced it.
It is also not a discretionary, moral power given to judges to right an abstract "injustice" - the plaintiff must demonstrate a legal basis for their claim, evidence the facts that support it, and defeat any relevant defences.
For the fact patterns in the question, the only cause of action that I can see is the 'free acceptance' of a benefit. However, this basis is not an established part of Australian law; there is no settled case law on it. The best authority we have is
Lampson (Australia) Pty Ltd v Fortescue Metals Group Ltd [No 3]  WASC 162, but that was only a strike-out motion where the judge recognised this type of claim as "arguable" and allowed it to proceed, but the parties subsequently settled so we don't know who would win that argument. It is accepted that if the defendant had requested the benefit in some way, then restitution is open, but silence, in the absence of a duty to speak, is not a request.
If the contractors in your situations were to bravely embark on taming and defining the wilderness of Australian law on 'free acceptance,' they would need to show the defendant:
- knew the work was being done,
- knew or should have known that it was not being provided gratuitously and that the plaintiff expected to be paid for it,
- did not take a reasonable opportunity open to them to stop the work.
This would be heavily fact-dependent on what the defendant knew or should have known and when they knew it. There are obvious issues where, for example, a tenant knows about the work but the owner doesn't, or the property is part of a strata development, where doors and windows are owned by the body corporate, not the unit owner. Similarly, where the owner first knows about the work when irreversible damage has been done to the property, as in your cladding removal example.
Notwithstanding, if 'free acceptance' is a cause of action under Australian law and if it can be demonstrated to have occurred, the plaintiff still needs to demonstrate in broad:
- The defendant has been enriched. Windows or doors that replace perfectly functional windows or doors are unlikely to have materially enhanced the property's value, whereas a new bathroom or kitchen might have.
- That the enrichment happened at the plaintiff's expense. This one is pretty easy.
- That it was unjust, in the sense of being unconscionable, for the defendant to receive the benefit without compensating the plaintiff for it.
- That there are no defences, for example, that good consideration was provided, or that the defendant changed their position to its detriment due to the plaintiff's mistakes such that restitution would be unjust.
- It may also be necessary to determine if the remedy is personal or proprietary. That is, is the owner liable, or does the liability attach to the property?