Not sure if it would be different with a UK-registered vessel, but for our USA-registered sailboat, Australia
granted us a 12-month "cruising permit", with the ability to renew twice, giving a total of 36 months. This allows the boat to be in Australian waters for 3 years, before duty and GST is required. I *believe* New Zealand
granted us, and the boat, a "9-month in, 9-month out" type of permit
. However, I also believe UK residents might receive preferred treatment in NZ.
Back to Oz, the boat can stay for up to 3 years tax-free (unless you sell it), but you may not be able to. Check into the visas available to you.