There are a number of different factors which go into the thread count of a bed sheet, and a few loopholes by which thread counts may be inflated. You can read about this here. If you’re sourcing your sheets from Australia only, then the main loophole to look out for is whether the sheet manufacturer complies with thread count based on number of threads regardless of ply-count of each thread (which is recommended but not necessarily required by authorities), or if they count each ply of each thread as a separate thread. In this way a 250 TC sheet with 4-ply weave could be misleadingly labelled 1000 TC.
Assuming that they count thread count the same way, thread count will tell you how tightly the sheet is woven. Generally speaking, the higher the thread count the heavier and softer the fabric feels. In our experience, 375 TC fabric has been more durable than 250 TC. However, higher thread count is no guarantee of longer lifespan or greater durability in sheets. Pilling can occur in any thread count sheets, but will be more likely in lower thread count. We have found that with 100% cotton sheets pilling is easy to remove, and so is not so much of an issue as it would be with polyester-cotton blend sheets. We have found from experience that pilling is more likely to occur under heavier sleepers.
There are factors that go into quality fabric than just thread count, so 400 TC sheets may well feel worse and last not as long as 250 TC sheets, depending on how they are made. One of these is thread courseness, which will depend on how the threads are spun or treated.
Unless you can find out from the manufacturers exactly how the threads are made and counted, the best test of quality sheets is to touch them.