As Pointman mentioned,
I also had the same intention as you but have been advised that as you will already be 20kms from the mainland, a gear breakage could be be fatal if you dont get back to the island, minimum precautions would be take a couple of flares and a mobile phone, support boat would be awsome but I would still take flares incase you were not being watched the whole time, when your in the water you will be near invisible so the flare will be handy.
Having said all that I would still love to sail over there, maybe Thompsons bay for the lesser experienced lime myself, I did see a guy 2 years ago blasting from the main jetty towards the north for about 500 meters gybe and return, I am sure he would have been blown back into the island at his further most northern reach, may be check it on google earth.
Cheers GT