it will be chilly in the winter ones (november-January) usually california actually doesnt get THAT hot, so i would do july, plus you are from australia right? its probably no hotter than what you are used to.
edit: assuming you dont work at one of the ski mountains in Australia or something.
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also, i dont know a whole bunch about the whole flight system, but when i traveled to australia i went from jfk to LAX into sydney on qantus. I dont know if they are an option for you, but for me they were superb. you probably know more than me in regards to the flying though.