This is yesterday’s reason:
Removing choice from workplace accident insurance
Allowing employers to choose to gain workplace accident insurance from companies other than ACC had been a great win/win. It gave employers real incentives to make their workplaces safer, and hence have cheaper premiums. In the short period of time the competition was allowed, premiums dropped massively, and accident rates also fell massively.
Labour re instituted a state monopoly, and to no surprise premiums have increased, with those for self-employed especially going up by 40%.