I don't know the technical detail of what information they can access - all I can do is share my experiences at LAX (and other US gateways).

The acutal processing is done at counters, very similar to SYD and other Australian airports. Assuming you have filled in your forms correctly (pay attention to this, it is frsutrating for all if you have to do them again), the chances are the agent will ask you a couple of questions along the lines of

'Why are you here'
'Where are you staying'
'What do you do in Australia'

Answer those honestly and you will be on your way very soon. It may not be a friendly process though. In all honesty, the worst part of the whole LAX experience are the huge queues. My last time, arriving on CX from HKG saw me wait 1.5 hours to get through.

I go through the US about 8 times a year for a variety of reasons and have only had a problem once. This was on an old passport, with lots of stamps from the Middle East which attracted attention and got me a secondary screening. That was a couple of years ago at YYZ. While rather embarassing, it hasn't done any lasting damage to my immigration file.

You do also have Customs to go through, but this is very similar to the Australian set up and should be familiar.

For peace of mind, about 6 weeks before your trip you might want to check the US Embassy web site to ensure that nothing has changed to entry requirements. And make sure your passport still has enough life!