I passed ATPL Flight Planning 3 weeks ago first attempt with 80%. I had to put in the hard yards and locked myself away for 2 weeks just doing question after question. What I found most helpful was Rob Avery's Practice exams. They are a similar length, content and allocation of marks as the actual CASA exam. If you can knock over an Avery exam in 2 hours and get around 85% you'll be set for the CASA exam.
I recommend doing the exam backwards, getting all the big marks out of the way first. That way if you start to run out of time you are only going to miss 1 or 2 markers and it is much easier to concentrate on knocking over the 5 markers when you feel relaxed rather than a mad dash in the last 30 minutes.
When doing practice questions keep a list of the mistakes you are making. I found that often times I would get a question wrong from a similar mistake I had made in earlier questions, so by making a list you can really drill into your head where you need to improve. Also make sure you re-do all the questions you got incorrect.
Make good use of your Highlighter in the 727 POH, it can help speed up how quickly you find information within tables. For example on the Altitude capability table I highlighted all the Westerly Flight Levels so I could quickly differentiate between the different Flight Levels. I would also highlight common routes on your ERC charts that come up in practice questions as this can save time looking back and forth between your ERC and calculations.
Finally, when doing your final practice exams put yourself under real exam conditions by having a countdown timer and only using equipment you will have in the exam eg. Doing your calculations with a graylead and rubber rather than a pacer on a set of 5 pages stapled together.
Best of luck and remember PPPPP (Prior Preparation Prevents Poor Performance)!
C & S