I am indeed a newbie too so not sure what advice of mine could help you (I assume you areon some kind of gap year or just taking time off to do a fair bit of travelling, which is an instant conversation starter with any HB) but it all depends on the scenario.
If it is anywhere in the DAYTIME then I guess, for example, if in a shopping district try asking for advice on good stores to buy clothes or other backpacking essentials. Trouble is, there is no perfect scripted routine that will work. I am afraid that you just have to be totally spontaneous and come up with something that is observational and relevant to the location. Unfortunately, I do not think anyone here can give good ideas for an opener yet. Unless of course you give detailed descriptions of the sort of locations where you have seen HB7+'s.

Mind you, I am sure there will be some posters from Sydney or elsewhere in OZ that could give good pointers on best places to try.
But one thing I have learnt is after approaching introduce yourself and give a light handshake. That itself is technically a form of kino!