New Zealand Law
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Is the main tenant a family member or are there just flatmates living there? Do you know if they have any agreement with the main tenant?
Is this for non payment of rent? What are the breaches? What does the agreement say?
If this is a contractual arrangement with family, then if you have cancelled the contract, which would be the effect of the 21 day notice, then if they will not leave you will have to apply to the District Court for an order for possession. It is a pity that the police will not act on the trespass notice, but I am not surprised. They are reluctant to get involved in issues which they see as family or civil disputes. My recommendation would be to apply for summary judgment for possession of the property in the District Court but you would probably need to get a lawyer to help you with the paperwork because there are particular forms and procedures which are needed.
However another alternative might be to try to mediate the issues. If you get an experienced mediator you might be able to sit down in a safe way and negotiate something which resolves the problems. It would certainly be considerably cheaper than going to court and enable the issues to be resolved with more dignity and less antagonism.