How do I complain about a Debt Collection Agency?

Hey ladies, gents.
Just after some advice.
Yesterday my wife got a call at home about our next door neighbour. I wont mention the organisation concerned but they identified themselves as ".... from Sydney". They started asking questions about our neighbour's personal circumstances, whether they'd moved out, whether anyone was home, why they weren't answering the phone etc. The main reason for calling was because 'they weren't answering their phone'.
My wife did the right thing and said "that's none of your business". They asked if we could pass their name onto the next door neighbour. We didn't disclose if the neighbours lived there but said yes we'd put the name and number in the letterbox and it was up to them to call you back if they still lived there.
I start walking next door, and the people across the road, and the neighbours on the other side of the house concerned are walking to the house with notes as well. 4 houses in our street were called about this particular house.
I rang this orgnisation late yesterday and asked "why are you calling houses in our street?", they said again that they'd tried to call this particular house several times with no answer. I said to them "but their affairs have nothing to do with us", they said that they normally use this technique to get people in touch with them. I asked them if they considered that they'd been not only invading our privacy, but were harrassing us with phone calls. I asked them what would happen if we kept getting phone calls from you and this guys response was initially "don't answer your phone" and then when quizzed said "just feel free to hang up on us".
Between our neighbours we now realise that this organisation is a debt collection agency from Sydney.
I would consider that a massive invasion of privacy now that we know the debt collectors are after one of our neighbours.
So back to the subject title....have I got grounds to make a complaint, and if so, who to?
Just after some advice.
Yesterday my wife got a call at home about our next door neighbour. I wont mention the organisation concerned but they identified themselves as ".... from Sydney". They started asking questions about our neighbour's personal circumstances, whether they'd moved out, whether anyone was home, why they weren't answering the phone etc. The main reason for calling was because 'they weren't answering their phone'.
My wife did the right thing and said "that's none of your business". They asked if we could pass their name onto the next door neighbour. We didn't disclose if the neighbours lived there but said yes we'd put the name and number in the letterbox and it was up to them to call you back if they still lived there.
I start walking next door, and the people across the road, and the neighbours on the other side of the house concerned are walking to the house with notes as well. 4 houses in our street were called about this particular house.
I rang this orgnisation late yesterday and asked "why are you calling houses in our street?", they said again that they'd tried to call this particular house several times with no answer. I said to them "but their affairs have nothing to do with us", they said that they normally use this technique to get people in touch with them. I asked them if they considered that they'd been not only invading our privacy, but were harrassing us with phone calls. I asked them what would happen if we kept getting phone calls from you and this guys response was initially "don't answer your phone" and then when quizzed said "just feel free to hang up on us".
Between our neighbours we now realise that this organisation is a debt collection agency from Sydney.
I would consider that a massive invasion of privacy now that we know the debt collectors are after one of our neighbours.
So back to the subject title....have I got grounds to make a complaint, and if so, who to?