Most importantly: ANSWER THE QUESTION ASKED.
If the question asks you to describe the role of ONE PM then only describe what ONE PM did. You won't get extra marks for doing TWO or more.
In History dates would be extremely useful to show either when things happened precisely e.g. when was the anti-communism bill introduced but putting in dates just for the sake of it won't get your further unless it is actually relevant and enhances what you are saying. e.g. saying that Menzies was the longest serving Post-War PM putting in the dates isn't all that necessary unless you were to want to show the period for some reason e.g. he was PM during the 50s when the US influence was increasing (PM from x - y).
Using historiography is good but not essential (just as it isn't essential at Modern History in the HSC but is in both Ancient and Extension) (From what I have been marking this year I wish kids would really understand this as some answers are just historiography and no attempt to actually answer the question asked).
If the question asks for a response covering a time period then clearly show the beginning, middle and end of the period e.g. technology since WWII would require a description of types of technology in the 40s/50s, talk about newer technology in the 60s, 70s, and so on - don't just jump from the immediate post-war period to the present but show that it has had stages to it.