Welcome to FlyerTalk!

It's true that you will only have to clear security once, but I must say, there is a decent chance of a misconnect on your itinerary. I fly JFK-SFO and SFO-LAX occasionally, and I have come to expect delays due to JFK and/or SFO and cancellations on SFO-LAX.

It's okay to book whatever flights are available on an award, but I recommend standing by for an earlier flight to LAX and, once at LAX, standing by for the next available flight to SFO. The reason being that if you miss your SFO-SYD flight, you'll lose an entire day getting home.

One thing in your favor is that UA also has a flight from LAX-SYD, so if your LAX or SFO flights appear to be late, UA could rebook you on that. (In fact, I'd probably ask at LAX if you could just take the LAX-SYD flight, anyway.)

If all your flights are on time, however, your connecting times are not a problem.