Leaves are healthy the new centre growth goes purplish underneath, bronze/yellow on top of leaves with (tightness?) distortion.

You might want to consider curly top virus or tomato spotted wilt virus as possibilities. Here are some links to help narrow it down:

Tomato Spotted Wilt fact sheet

Tomato Problem Solver

disease management in tomatoes

digital diagnosis

Would suggest that you talk to your local extension service or local agricultural agent (or whatever you call it in NZ) to find out what what the incidence of spotted wilt and/or curly top is in your area.

Hope this gives you some ideas.

Spray regularly with daconil.

Won't help with systemic diseases which is what your sounds like.

I'd pay attention to the Spotted Wilt suggestion more than Curly Top, but check them both.

Carolyn