Police believe an Auckland man was neither intending to commit a crime nor carrying any weapons when he was killed at the property of another man.
David Cunningham was killed at the property in Ramarama in the Bombay Hills after a man in his 20s who lived there confronted him about 1.30am on 27 August.
Police say Mr Cunningham, who was unarmed, was stabbed and died before the police arrived.
They cannot lay charges because they are not able to prove the man at the property wasn't acting in self-defence.
Inspector Dave Lynch said they have been unable to prove why the 40-year-old was at the property, but don't believe he had any criminal intent.
He said the case was reviewed by the Auckland Crown Solicitor and could be reopened if more information comes forward.
The Law Society said it can be difficult for the Crown to disprove an argument of self-defence.