Welcome along fellow Kiwi..
With mine i changed to a 3" system right through the car with a muffler half way to make it quite. Was way to loud with just the end muffler on.
I brought a stainless "Promuff" muffler for the rear with a 3" outlet .
Car has a nice "deep" sound to it and not a "bark" like you get with others.
If you are doing the exhaust try and get a good air cleaner(K&N etc).
All up it cost $500.00