Whatever else it may be, this isn't a breach of contract.
If the contract says Qantas can unilaterally change the terms at any time, they can.
The fundamental premise of contract law is that the parties are free to enter into the contract or not.
Accordingly, despite limited restrictions, the contract can contain such terms as the parties agree it should contain.
If a party doesn't like the terms, they are free to walk away.
This fundamental premise is somewhat unrealistic, particularly in consumer and employment contracts.
Hence the limited restrictions in consumer and employment law and more recently in "unfair contracts" law.
However, off the top of my head, I can't see this falling under any of those restrictions.
As to whether this deprives the member of the entire or a sufficiently substantial benefit of the contract so as to render it of no value to them, I think Qantas would have a good argument that entry to the Qantas Club and enjoyment of the services it provides is the principal benefit of Qantas Club membership. Other benefits such as priority check in and (formerly) priority baggage are probably best characterised as ancillary benefits.
This is not a subjective test. It does not matter what the OP says their reasons for joining were. It is an objective test.
This means a court would look at the terms of the contract and what lawyers call the "factual matrix" around the formation of the contract to determine the intent of the parties. This may seem bizarre but you can't simply ask the parties what their intention was because their evidence would be self-serving. They would say whatever they needed to say to make their case. Either disingenuously or simply because they are interrogating their memories with the benefit of hindsight in the context of the relevant dispute.
In this case I don't think the OP has much chance of receiving any compensation but it is always worth a try.
They may end up being given 5000 QFF points to STFU.
