Agree with your adversary quickly.. so tell them you are happy to pay on the condition they provide copies of any contracts you have with them. They also need to provide you with the deed of assignment. This is required from the person you owe money to, to the debt collector.. there are laws in each state that cover this, plus a high court case. Ozdil V Vresky if I recall correctly
Once you ask them to provide the proof of their claim, they have 14 days to provide it under the National Consumer Credit Protection Act 2009, Division 2,Section 185. If they don’t provide it their chances of collecting from you are slim to none.