It doesn’t matter where the owner lives because the debit is not against the owner but against the lot. Also you should be charging interest against this debit of 10% per year to the lot owner.
I feel you need to start proceeding against the lot owner in the courts (with a solicitor who specializes in debit recovery). All costs that the Solicitor incurs are claimable against the lot owner.
If the lot owner ignores all attempts to communicate the courts will eventually decide to sell the property and deduct all charges and costs (including the Strata debit) from the proceeds of the sale before forwarding the balance to the lot owner.
Even if the owners sells the property privately the strata debit remains with the new owner and the new owner will have to cover the 15K debit (plus costs).
One of the jobs of a conveyancer is to make sure their are no debits on a property their client is buying and if their is withhold enough of the sale price to cover the debit.
Shame you are not in QLD as here the interest rate is 30% which really makes their eyes water.