Part of your confusion may come from the fact that there were two separate actions.

On March 21, the government issued an Order in Council, [Arms (Military Style Semi-automatic Firearms) Order 2019
(LI 2019/55)](http://www.legislation.govt.nz/regulation/public/2019/0055/latest/whole.html), with the following short text:


> For the purposes of the Arms Act 1983, the following firearms are
> declared to be military style semi-automatic firearms: 
>
> (a)
> a semi-automatic firearm that is capable of being used in combination
> with a detachable magazine (other than one designed to hold 0.22-inch
> or less rimfire cartridges) that is capable of holding more than 5
> cartridges: 
>
> (b) a semi-automatic firearm that is a shotgun and that is capable of
> being used in combination with a detachable magazine that is capable
> of holding more than 5 cartridges.

The Arms Act 1983 already banned "military-style semiautomatic firearms".  As an Order in Council, this measure could be taken by the government directly without waiting for Parliament to act, and was apparently meant as a temporary stopgap.

On April 11, Parliament did pass legislation, the [Arms (Prohibited Firearms, Magazines, and Parts) Amendment Act 2019](http://www.legislation.govt.nz/act/public/2019/0012/latest/LMS181180.html), whose (much longer) full text can be found at that link.  This act also revoked the previous Order in Council which was then no longer needed.