New Zealand Law
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In these circumstances you can apply to the Tenancy Tribunal to cancel the lease. The grounds for doing so are under section 66 which says
Reduction or termination of fixed-term tenancy-
"(1)On application by a party to a fixed-term tenancy, the Tribunal may make an order reducing the term of the tenancy by a period stated in the order, and making such variations in the terms of the tenancy as are necessary because of the reduction of the term, where it is satisfied that, because of an unforeseen change in the applicant's circumstances, the severe hardship which the applicant would suffer if the term of the tenancy were not reduced would be greater than the hardship which the other party to the tenancy would suffer if the term were reduced."
This would be grounds because of the physical threats and the other actions. Your landlord also has obligations to you to protect your quiet enjoyment of the flat, and by failing to evict the neighbours he has breached the lease. So you have two matters to raise, cancellation of the lease and damages for failing to protect your quiet enjoyment of the unit.