If your lucky you will get some 15-20 south easters in the morning and maybe a seabreeze in the late arvo.
I would go for 90 litres plus slalom gear, but I've never used formula. Unlikely to need less than a 6.5 sail. South east is offshore so you need big enough gear to get back if the wind drops off (not an area you want to be swimming your gear back in).
Keep an eye on the bom site for what the wind is doing during the day. The wind doesn't change much for the next few months.
www.bom.gov.au/products/IDD60901/IDD60901.94120.shtmlIf you do get to Darwin get in touch as I live about 50k's down the track.