Getting it legal for the road is also alot more difficult, normal certification process does not apply. You need to go through MSNZ, become a motorsportclub member etc then once you have all the documentaton + proving froma reputable builder who built the cage (prebuilt overseas ones as your trademe ones are pretty much nigh impossible to legalise in NZ as you have to prove what the metal is etc etc)..before you even get your car back on the road. I went through this whole process on my track car, however my builder was able to ensure he all the supporting photos of welds/details to send off which made life a boat load easier.
...reason for difficulty, as above without the other supporting racing equipment you will come off worse in a road crash and really the rules are designed for those that want to travel from and to events in their race car. Cages are designed to take high impact hits and be the last line of defence in a serious crash, if your head hits a bar without a helmet it is bye bye head...it hurts enough just accidently hitting your head on a bar when you get into a caged car haha.
Also makes you a massive pull me over target and if not up to spec with MSNZ licence and alike then you will get hammered with fines/get stickered.
Best bet is find a good reputable cage builder, discuss with them what you want, your goals etc and then go from there. They will recommend a design that will get through certification/MSNZ rules and will ensure you know exactly what needs to be done.