In order to answer your question, we made sure to check with a Rav who has flown back and forth to Australia. You are correct, on the way to California you can go to the mikveh Wednesday night. You will in fact have counted seven days, as you will have experienced seven sunsets. (A day is defined as the period from one sunset to the next. When travelling between time zones, this may be longer or shorter than the standard 24 hours.) On the way back from California to Australia, if you did a hefsek taharah on Monday (before sunset), you would go to the mikveh on Tuesday night as you would need that “extra day” to have seven sunsets.