Just adding to what you said....
If you are turning over $75k or more per annum you are required to be registered for GST, and hence collect GST and submit BAS statements.
If you are turning over less than $75k per annum, GST is optional but you can't just simply "collect" it, you need to be registered for GST in order to collect it. There are obviously benefits to being registered for GST regardless of your turnover, but it comes with the hassle of extra book keeping, paper work, and submitting BAS statements. All this is made easier with simple accounting software though.
To the OP, an ABN is simply your registration of a business. It is required for running a business, which is exactly what you'll be doing if you contract directly to the security company. You don't just simply give them your ABN and let them sort out the rest, your ABN is not a bank account. You need to submit an appropriate invoice to your clients to be paid.