Nah, like those Lee key drive slugs, or Lyman giant airgun pellet slugs (I've only read about them in the forum, never seen them). Added a picture, that might make more sense!
Also, nope, they're not illegal nor are sabot shotshell rounds, can read here:
Arms (Prohibited Ammunition) Order 2019 (LI 2019/137) (as at 20 December 2019) Schedule Ammunition declared prohibited ammunition – New Zealand Legislation
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