timmmay: Can't help with the solution, but I've used Fred Chapman Garage doors in Porirua and have been happy with their service.

Jiriteach:

 

Recommend Fred Chapman as well - https://www.garagedoorswellington.co.nz/
Its actually a resonably simple process for them to replace the springs. Can take ~ 30 mins or ~ 2 hours if they are to replace the motor.

 

Thanks for the recommendations.  Called these guys on Tuesday morning and by 14:30 on Wednesday the door is usable again.  Both springs have been replaced and one of the cables, just waiting on the other replacement cable.  

They'll send the invoice once the whole job is done but speaking to the bloke yesterday he reckons it'll be about $400 all up.

I'm very pleased.  When I saw the initial busted spring I had visions of several thousand $$$ and not being able to use the garage for weeks.