If going North from Perth (Freemantle) you would leave in May and travel North to Darwin. Going East from Darwin you really don't go before Late September or October and crossing the Gulf of Carpenteria is best after the first part of November. Heading South for the top of Cape York you can get light winds and NE winds from November. Remember there are stingers crocodiles etc to consider. Stingers come out sometimes as early as October in Queensland
Trust me when I say it's foolish to cross the Gulf of Carpenteria before November. But maybe your skipper
plans to go down in the gulf. We could talk for hours just on gulf crossings. I did the trip from Cairns to Darwin and back last year.
If going South from Perth I believe you would leave in September but I'm not 100% sure when is the best month to cross the bight.
Hope that helps.
I would say avoid the top of Australia
December through April the wet season, and cyclones come earlier in the west coast
before the east coast
. Maybe someone else can add more about the Southern route
. I've only ever sailed North from Perth.
Vaya con Dios