How? Water are in fish tanks.
If you are looking for Zooplankton go and check the millions of tons of water being dumped by Ships?
As mentioned, no system is perfect but importing fish has a far lower risk than importing Bananas vegetables etc.
As for testing fish , there is more than just paying for a test, there is the also additional MAF visits etc,.
And no doubt as the years go by the cost of testing will increase and so will the high risk fish list.
MAF can test all it likes, however should be covered by them.
Far more powerful actions would be to improve education, like big signs in stores to say things like. wash hands etc, no dumbing of fish etc,
So far quarantine has worked so way add more costs to the hobby for things that might happen.