That depends. There might be a bit of drift. If one hive has a problem & the others rob it out then disease can spread.
If one hive has AFB then they can all be exposed. AFB is the worst case scenario. Other diseases are manageable but still a pain in the arse.
I'm speaking from experience. I lost all of my 8 hives to AFB in August. I didn't pick it up early enough in the autumn & it was right through them in the spring.
Failing to identify it early was my fault. I am a lot wiser now.
I don't know how they picked it up. Possibly from robbing out a hive in the scrub around my property. Looking back a lot of the practices that I used could have aided the spread eg putting extracted stickies back into any hive.
Allowing bees free access to honey in the open is a bad practice & I have never done it.
Unless you have more than one apiary you should try to minimise the intermingling of hive parts & material between those hives.
Bernsad, with the greatest of respect to our American friends & particularly JP you should consult the several reliable sources available in OZ. In particular the QLD & NSW DPI websites. Forums should not be the only source of your information.
Both of these sites have heaps of reliable info.http://www.dpi.qld.gov.au/27_124.htmhttp://www.dpi.nsw.gov.au/agriculture/livestock/honey-bees
There may be info more relevant to Victoria however you can chase that up. I'm a cane toad living in NSW so I'l stick with those states.