Fitz, the very first thing that would happen if you offered different rates to different companies is that companies would try whatever they could to avoid reporting accidents or injuries. Why would they want to cost themselves more money if they can bully a staff member into lying that they cut their hand off over the weekend in the shed working on their boat, instead of at work on some dodgy piece of machinery with no safety guards.
Currently, ACC does not penalise anyone if someone injures themselves. They refer to themselves as a 'zero blame'. What you are suggesting is that ACC spend time to investigate the
causeof each claim. This would
increase their costs. Why do you want them forced to spend more money?
(nb. This doesn't mean a company is not investigated if someone gets fucked up at work - but that's the Department of Labours domain, and that won't be changing either way).
ACC already offer a discount on your levies, at 3 levels (iirc, it's about 2%, 4% and 8% but I can't remember exactly), based on a 2 yearly audit under the Workplace Safety Management Program. They have third party auditors come and look at things like what your doing to minimise hazards and control injuries, the training you're providing, and any 'extra' things you might be doing to help your staff out.
ACC also put a lot of effort into trying to make sure they never
have to help people. They provide training to companies, and things like outlines for them to put Health and Safety programs into place. They've put a lot of effort into 'keeping fit and healthy' initiatives such as
ActiveSmart. I don't expect any of their competitors to do any of these things with their shareholders money.