There are many providers around, all of whom have plans which will probably be quite sufficient for your average visitor. Typically, each will have an unlimited domestic calls/SMS option plus 2GB to 5GB data for around $30-$40 for 30 days, and another option of around $10-$20 for 30 days with calls costing around 10c/min and data around 5c/MB. All depends on whether your visitors are likely to make many local calls/texts, or mainly want the SIM for data.
Non-exhaustive list includes Kogan (as above), Aldi, Amaysim, Boost, Woolworths as virtual network operators, and of course, the carriers themselves (Telstra, Optus, Vodafone) with typically higher pricing for the same services.
My recommendation would be to choose a provider where you can physically buy the SIM from a store, and email the SIM number/activation code details to your visitors, so that they can activate the SIM themselves online, using their own ID, before they arrive (usually need to supply two forms of ID, e.g. drivers licence, passport number, etc).
You could, of course, activate the SIMs for them using your ID, if you trust them not to do illegal things with the SIM.
From my experience, Amaysim, Woolworths and Aldi SIMs are reasonably easy to get from 7-11 (Amaysim), Woolworths and Aldi stores, and I believe Boost SIMs are available at Dick Smith. Kogan requires a mail order, and an unpredictable wait for delivery.
I've had no problems activating Amaysim and Aldi SIMs online, with just supplying my licence number, but YMMV.