Making smacking a criminal offence will not support parents in learning not to smack if they are not supported in alternatives.
Making smacking a criminal offence will not stop the terrible abuse.
So lets start with the easy one. This is easy because we are a thinking society and we can do this without going straight to punishment. Let's start satellite groups attached to kindergartens where parents meet to workshop "What are the alternatives to smacking" "What to do when you are about to smack." "What are the benefits." "What is the future?" "Will we get a whole bunch of spoilt out of control kids?" or "Will we promote a community of learners who are learning to act for intrinsic reasons?"
Abuse. I think one place to begin, is with the adults who live next door, the other adults in the house, the relatives. Let's empower them to tell someone when they think 'it is not right'. Empower them to know the child/baby deserves/has a right to a physically and emotionally safe home. This may be the type of abuse they grew up with too so this must be difficult.
Who can they tell. Public health nurse, kindergarten teacher, doctor, Pita Sharples, MP, Christine Rankin, Cindy Kiro, kaumatua, - tell someone who will know what to do, or find out what to do.