You ask about how to introduce yourself to prospective interviewees. Why not say: "Hello, my name is * and I'm a freelance journalist. I'm writing an article for XXX publication on the topic of YYY, and as you're a very respected expert in that topic - even Wikipedia says you're Australia's best - I was wondering if you'd be kind enough to give me a few minutes of your time to ask you some questions so I might quote you in my article". This is all true. If he/she questions whether you've been commissioned or you're doing it on spec, you'll have to answer truthfully. But if you sound confident, friendly and knowledgeable about the topic - and if you ask questions that are intelligent and well-informed - you'll get your interview.
If you sound amateurish, shy, halting, time-wasting or uninformed
you'll get the old brush-off.
Whatever you do, don't lie and don't exaggerate. Don't beg ("Take pity on me, I'm just a poor beginner"). But do smile while you talk, do be charming, do LISTEN, and do be quick.