Hey Alex
Thought I’d chime in a bit here and answer the questions for you 
Easter is from April 2-4 next year, so wont coincide with anything in March.
The Grand Prix is usually on the last Saturday of March but dates are not confirmed for next year yet. That’s in Melbourne.
As far as horse racing events go, the only ones I can think of are the Adelaide Cup, which will be on Monday 8th March, and the Sydney Autumn Carnival which is starting on the 10th of April next year to avoid conflicting with Easter…
The only other notables: Cheltenham racing in the UK from 16th-19th and the A-League grand final on the 21st (A-League is our Australian Soccer League).
I wasn’t able to find anything that is occurring during the 11th-14th March (Thurs-Sun).
If you need anything else please let me know. I’m willing to help out again this year
Cheers
Renee
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PS – March is probably the best time of year to come to Oz because it’s still hot. Weather is awesome. Any earlier than that would probably have anyone coming from a cold, sunless place leaving with second degree burns. The hole in the ozone layer is right on top of us so don’t forget your sun cream.