The following is from the NZDF Medals website.

The medal is for those who have served in the military for three years or more since the end of the Second World War, including those who did Compulsory Military Training (CMT) or national military service. It will recognise the unique requirements of military service. These requirements include: - commitment to service of the Crown (represented by an oath/affirmation of allegiance),- liability for operational service subject to military discipline and lifestyle,- and imposed constraints on employment conditions and personal freedoms. In broad terms, the basic eligibility requirements by type of service for the award of the New Zealand Defence Service Medal are:

a. For Regular Force (Navy, Army and Air Force) personnel: three years service in the New Zealand military after 3 September 1945, or completion of initial engagement which began after 3 September 1945. The three years can be accumulated and do not need to be continuous.

d. For National Service (Navy, Army and Air Force) personnel: completion of obligations under the National Military Service Act 1961 and Amendments.

e. Members of the British Commonwealth Occupation Force who completed 12 months service or their obligations for service in Japan would also qualify. Similarly the 12 months or more of military service in New Zealand or overseas by Second World War personnel still serving on or after 3 September 1946 would qualify these personnel for the award of the New Zealand Defence Service Medal.

Service in categories a. to d. above can be accumulated, if required, to reach a total of three years qualifying service for award of the medal. In special cases certain shorter periods of service will also qualify for the award when there are deserving circumstances. For example, when a service person dies while in service, is medically discharged due to a total and permanent disability, before completing three years service, or is medically discharged for any other reason after completion of 12 months service. The medal will also be awarded to those personnel discharged through no fault of their own when their service was no longer required as a member of the Armed Forces.

The online application form is here:

https://forms.mil.nz/medals/nzdsm-application-form.aspx