In New Zealand

You must first report the incident to the local police as soon as possible and obtain a police report in the cases of a stolen passport.

You then need to tell the High Commission, as this will reduce the risk of someone stealing your identity or attempting to enter the UK on your passport. To get your passport cancelled, send or take a copy of the police report along with a completed LSO1 form to the:
Regional Passport Production Centre
British High Commission
44 Hill Street
Wellington 6011 

If you are in the UK, contact the Identity and Passport Service on their Passport Advice Line 0870 521 0410 for guidance.

Once you report your passport as lost or stolen, the passport is electronically cancelled and is no further use. You must not use it to travel, as this may cause the passport to be impounded by the Immigration Service on arrivals, and if attempting to travel overseas you could face deportation or arrest. Using a cancelled passport as any form of identification may be regarded as a deliberate attempt at fraud.

When a passport has been lost or stolen, you can apply for a replacement using Form C1  and notes (for an adult 16 years and over) or Form C2  and notes (For a child under 16 years). You need to provide details of the lost passport and relevant documents if available. Enclose the police lost property report, plus identification and two recent identical passport photographs - one certified by the person who countersigns your passport application form.

You also have to provide details of the person who countersigned your previous passport application, and also the address where you were living when the lost/stolen passport was first issued.

There will be the usual passport fee. 

A restricted passport may be issued to enable further enquiries to be made about the lost or stolen passport. These are normally valid for no more than a year.

Form R01 should be used by law enforcement agencies or consular staff to report details of passports that have been recovered, where identity fraud or related crime is known to have taken place.

What if I later find the passport I have reported as lost or stolen?

If you later discover the passport that you had initially thought was lost or stolen, you must return it to the nearest British High Commission, Embassy or Consulate as soon as possible. If you are in the UK you should return your passport to the nearest Identity and Passport Service office.

Returning the passports of deceased persons in New Zealand

It is important to cancel the passport of a deceased person so that it cannot be used illegally.

You must return the passport with a copy of the death certificate to the British High Commission in Wellington, along with a completed copy of the form 'What to do with a passport when the passport holder has died'

The form allows you to choose for us to send the passport back to you or dispose of it. If you do not choose an option we will dispose of the passport.

When we receive the form we immediately cancel the passport.