every school is very different.
depending on your school your half yearlys will usually be in term 1 ( sometime ) and trials in term 3 ( sometime ) . Thats as accurate as you can get ( for now anyway )
but in 2009 when I did my hsc , im pretty sure I remember we had half yearlys 4wks into term1 and trials 2wks into term 3
most schools have their trials early in term 3 so the exams can be marked and returned quickly and people can spend the rest of term 3 revising ( and all the bludges that put a whole weekend of effort into preparing for their trials will have time to do some actual preparation for the hsc exams ) , but as shown by others posts some schools do have relatively late trials