Check the tube size is correct. I saw a tube the other day that had the right size stamped on it, but when compared to the original tube was a different size. Typical in Oz.

Next time you have the tyre off, get a small ball of cotton wool and run it around inside the tyre. It might snag something that you can't see. With a texta or pen put a mark on the tyre and align it with the valve whenever you put it on. This will help to locate if the punctures are appearing in the same place if it's the tyre.

As a last resort, check the rim for cracks. This is sometimes the cause of a lot of unexplained punctures.