Nobody should have to put up with this level of abuse and threats. It sounds to me that you have sufficient evidence to go to the police and make a claim of harrassment.
Also look here https://www.environment.nsw.gov.au/pollution/noise.htm
Noise pollution can be reported to your local council and to the Police assistance line on 131 444.
See also Jimmy’s note re Noise Abatement orders.
Every time one of you receives a threat immediately report it and every time that there is a party going on into the night report it. This way you will protect yourself because these people are mindless bullies and you have to stand up to them. Keep a diary of every single event as you may one day have to go to court over this and then you will be well prepared.
Good luck