If people aren't going to other people's homes, or to work or to school, how is it being spread?? THAT'S the point of a lockdown.
Are you a bit dim? Unless you fancy living without food, water, electricity, medical care and other things, a lot of workplaces as well as places like hospitals will have to be open during lockdown, no matter how "strict" it is. Many of the people working there will have dependent kids, so either some kind of childcare center provision or a skeleton school service for essential workers' children will have to stay open too and many children will go there. People will also spread COVID about in their own households, to other family members. People in hospitals can spread it to each other and to HCWs as well.
It doesn't matter how strict the lockdown is and it doesn't matter how carefully people try to adhere to the rules-you cannot shut down all possibilities for transmission, which is why it is very hard to completely eliminate the virus through lockdown unless you move very quickly, before the virus has become seeded in households and essential workplaces-and even then, there will be a very large element of luck involved. NSW was probably unlucky this time. But it was inevitably going to happen at some point.