Never hurts to ask, but Emirates bases your access on your onward departing flight, regardless of connections. You could still go in if you had Platinum with Skywards or Qantas, if somebody guested you in, or if you paid. Rates are on the Emirates website. If you technically qualify for the business class lounge through status (or likely even Qantas Club), you can pay a reduced 'upgrade fee' to use the first class lounge rather than business class. Otherwise, paying for the first class lounge is pretty pricey.