Hi and welcome to the Redback garrison.
Head on over to
www.whitearmor.net this is the stormtrooper detachment.
There are a couple of very good builds for Phasma over there.
One is based and modified off the Anovos First order trooper and I think the other is full custom built, there is however a Captain Phasma conversion tutorial by a forum member ladyinwhite, go check it out.
You have picked a costume that will be challenging indeed as there are long wait lists for this type of armour and they currently don't ship direct to Australia nor are they selling to non 501st members. (This will change in the near future)You would need an shipping contact or an on-forwarding service.
I am also uncertain if there is a full costume reference set up for approval for 501st as yet.
Do plenty of research and enjoy the ride.
P.S. There are plenty of members in your area so you will have good local help.