still not sure if i actually owe them this money or not but i have paid

I am in my 15 weeks of pregnancy who stay sick most of the times , and paying $650 is just not as easy as they think for people with normal income ., i received a call from collection agency and the representative threatened me to enter into an immediate payment arrangement or she will put default on my credit history and do wage garnishing and to avoid that we decided to start paying this off and its been 2 months and they have failed to provide me any statement and documents and i am still not sure if i actually owe them this money or not but i have paid them $110 already to avoid and further action. It is so hard to get hold of centrelink and collection agency don't have any proof and they have refused to provide any document or lodge my dispute and they say i have to do that with centrelink directly . The letter i received from centrelink states i owe them money for 2015-2016 but last payment i got from them was in dec 2015 they did not pay me anything in 2016 .
I am happy to pay but all i want to see is proof and my statement of how and when they have overpaid me ,
It has effected us financially and emotionally , my husband has to do extra shifts on weekend to pay this and we don't get personal time , resulting in stress and frustration . i stay sick most of the times due to pregnancy and have been to hospital few times and i have to deal with all that alone with my 2 year daughter .