I used to work in a pharmacy and shop bought tests are about 98% accurate. Doctors don't usually bother testing again if you get a positive at home, because they are so good.
There is some instances where you can get a false positive and this can sometimes occur when an egg has been fertilised but for some reason - usually because it is defective in some way - aborted itself and reabsorbed. This is why it is best to wait until you miss your period before you test because that gives the egg time to implant and begin to grow which means it is more likely that a positive test is accurate. Also sometimes if you test too early the test can be a bit faint, but the hormone it tests for increases really rapidly so if you wait a few days then test again it should be more definite.
I would however suggest purchasing a test brand that is fairly well reconised such as clear blue (I think you can get that in Aus, just for peace of mind.
Hope that helps
Leanne, NZ, Ella age 2 & Sam age 6months