Can a citizen of New Zealand return home on an expired NZ passport, if they also possess a valid UK passport?

The official advice is no, you cannot enter New Zealand using an expired New Zealand passport.

In the case of NZ and Australian passport holders or NZ residents/Australian permanent residents (with current travel conditions), your passport must be valid for enough time to allow you to travel to New Zealand (ie, it must be valid on the date you are entering/leaving NZ).

http://www.dol.govt.nz/immigration/knowledgebase/item/1166

[I can see that you might claim that you are a permanent resident and therefore your British passport should suffice as it is valid on arrival. Usually, however the meaning of "permanent resident" is restricted to a non-citizen resident whose foreign passport contains a residency visa.]

TIMATIC, the information used by the airlines, also states this restriction clearly (full text given at the end of my answer).

- Passports and other documents accepted for entry issued to
nationals of Australia and New Zealand must be valid on
arrival.

However, it does say that you can enter New Zealand by using a "Certificate of Identity issued by New Zealand". Whether you would be entitled to leave New Zealand on such a document is not specified.

Passport Exemptions:
(...)
- Passengers with a Certificate of Identity issued by New
Zealand.

Some commentators have suggested that you will ultimately be admitted because the immigration authorities have no right to deny their own citizens entry. This may be true (I have not investigated the relevant New Zealand law), but it is inadvisable, in my own opinion, for several reasons.

The first is that, without an onward ticket out of New Zealand, your air carrier may refuse to fly you back to New Zealand on either a British passport or an expired NZ passport. The second reason is that, on arrival, you may be detained for some time in discomfort while your credentials are investigated. The third reason is that you have not yet begun your trip, and to exit the country you will need to present a passport. Either it is your expired passport, which should not be accepted, or your lack of recent entry stamps in your UK passport might raise eyebrows.

Of course it is quite possible that no one will notice that your passport is expired and your trip will be smooth. But I think it is wise to update the document if you can.

Full information from TIMATIC.

Passport Information

Information as of  26APR16 / 1142 UTC
New Zealand (NZ)

Passport required.

Passport Exemptions:
- Passengers with a Seaman Book.
- Passengers with a Certificate of Identity issued by New
Zealand.
- Passengers with an identity card issued to stateless persons
and refugees if approved by foreign representations of New
Zealand at time of visa issuance.
- Passengers with a Laissez-Passer issued by the United
Nations.
- Passengers with a Macao (SAR China) Travel Permit.
- Nationals of Timor-Leste with a Temporary Travel Document
issued by Timor-Leste.
- Passengers with an International Committee of the Red Cross
(ICRC) Travel Document.

Document Validity:
- Passports and other documents accepted for entry must be
valid for a minimum of 3 months beyond the period of
intended stay.
- Passports and other documents accepted for entry issued to
nationals of Australia and New Zealand must be valid on
arrival.
- Passports and other documents accepted for entry issued to
nationals of Argentina, Brazil, Canada, Chile, China
(People's Rep.), Colombia, Cuba, Fiji, France, Germany,
Greece, India, Indonesia, Iran, Ireland (Rep.), Italy,
Japan, Korea (Rep.), Malaysia, Mexico, Netherlands,
Pakistan, Papua New Guinea, Philippines, Poland, Russian
Fed., Samoa, Singapore, South Africa, Spain, Switzerland,
Thailand, Turkey, Tuvalu, USA, United Kingdom, Vanuatu,
Vatican City (Holy See) and Viet Nam must be valid for a
minimum of 1 month beyond the period of intended stay.
- Passports issued to permanent alien residents of Australia
must be valid on arrival.
- Passports issued to residents of New Zealand must be valid
on arrival.

Admission and Transit Restrictions:
- Chinese Taipei (on the cover: Republic of China Taiwan)
diplomatic or official passports are not recognized for
entry.
- Admission and transit refused to holders of Travel documents
issued by Somalia.
- Admission and transit refused to holders of Iraqi "S" series
passports.

Crew Members:
- Operating Crew: passport and Crew Identity Card required. A
crew declaration form must be completed.
Positioning/Deadheading Crew: Passport required and an
arrival a form must be completed.

Military:
- Passport is not required, provided travelling on duty and
holding a Military ID Card (with a photo) issued by the
government of the country whose Armed Forces they belong to.
The ID Card must be accompanied by the movement order.
Note: a member of the armed forces of any country is exempt
from certain requirements if they are in New Zealand at the
request of, or by agreement of, the New Zealand Government
as part of their duty or employment.

Minors:
- Minors entering New Zealand may be included in the passport
of the parent, who is travelling with them.

Warning:
- The official three letter country code in passports issued
by Zimbabwe in ZWE. However, for a limited period, passports
were issued with the three letter code of ZIM. These
passports are still in circulation and will remain valid
until their expiry date.

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AUTHORIZATION CHANGES