Source: https://ihl-databases.icrc.org/customary-ihl/eng/docs/v2_cou_nz_rule70
Timestamp: 2019-04-18 19:43:34+00:00

Document:
It is prohibited to employ weapons, projectiles and material of a nature to cause superfluous injury or unnecessary suffering. A weapon causes unnecessary suffering when in practice it inevitably causes injury or suffering disproportionate to its military effectiveness. In determining the military effectiveness of a weapon one looks at the primary purpose for which it was designed.
New Zealand, Interim Law of Armed Conflict Manual, DM 112, New Zealand Defence Force, Headquarters, Directorate of Legal Services, Wellington, November 1992, § 509(2) (land warfare) and § 616(2) (air warfare); see also §§ 510(1)(a) and 707(2) (naval warfare).
Examples of such weapons include such weapons as lances with a barbed head, irregularly-shaped bullets, projectiles filled with broken glass, and the like. The scoring of the surface of bullets, the filing off of the end of their hard case, and the smearing on them of any substance likely to inflame a wound, are also prohibited. Generally speaking, weapons which are agreed to cause unnecessary suffering are home-made weapons or unofficial modifications of weapons issued through normal channels.
New Zealand, Interim Law of Armed Conflict Manual, DM 112, New Zealand Defence Force, Headquarters, Directorate of Legal Services, Wellington, November 1992, § 510(1)(a), footnote 44; see also § 1704(2)(e), footnote 37.
Lastly, the manual states that “employing arms or other weapons which are calculated to cause unnecessary suffering” constitutes a war crime.
New Zealand, Interim Law of Armed Conflict Manual, DM 112, New Zealand Defence Force, Headquarters, Directorate of Legal Services, Wellington, November 1992, § 1704(2)(e).
It was difficult to determine criteria for unnecessary suffering, except in the case of the indiscriminate use of weapons. One should not fall into the error of giving preference to weapons that killed cleanly rather than to weapons that wounded but did not kill.
New Zealand, Statement at the CDDH, Official Records, Vol. XVI, CDDH/IV/SR.2, 14 March 1974, p. 18, § 5.
New Zealand, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, § 69.

References: § 509
 § 616
 § 510
 § 1704
 § 1704
 § 5
 § 69