The oustanding amount is anywhere between $20k and $50k so not exactly peanuts.
Have you considered the possibility of fraud by the previous strata manager?
The amount is large enough that there needs to be some serious questions asked about how the situation got so bad and where the money is.
A forensic accountant needs to go through the accounts left by the previous strata manager and their bank accounts. If evidence of fraud is found then a referral to police should be made. If the previous strata manager was committing fraud against your OC, then they’re doing it to other OCs.
How was payment supposedly made? Cheques and bank transfers are easy for the unit owners to prove they’ve paid. The untraceable method of cash payments without immediately issuing a receipt supports the possibility of fraud. If a receipt was issued but not accounted for then further inquiries need to be conducted.
Finally, time to have a bank transfer/BPAY only payment system. Easy to track and automatically issues receipts, shows payment status.