It sounds like there was a court hearing, notice was sent to your last known address but because you moved, you did not receive notice of the hearing. The hearing went ahead without you and the other side received a favourable judgment (most likely a Default Judgment). They are now asking you to pay their legal costs in relation to the hearing. Is this correct?
First, ascertain whether you have a default judgment against you. Ask the other side for a copy of the judgment, or call the court to enquire.
Second, are you disputing the actual (primary) debt? If you are, you will need to set aside the default judgment. You will need to apply to court for this. If you are not, meaning, (i) you did in fact owe the other side strata levy, and (ii) they had a right to sue for this in court (check your agreement to see if they are able to go to court without preliminary notice procedure), then you will need to reimburse their legal costs (iii) if they win.* On the default judgment, the court would have made an order as to costs. Check your default judgment and see what this is.
The normal costs order is that the losing side pays the winning side reasonable costs (legal fees, disbursements, filing and service fees etc). You can ask them for a detailed itemised list of fees and expenses. If you believe the costs are unreasonable, you can dispute them in court (known as "taxation of costs"). This is the process where the taxing officer goes through each item claimed, match this against the "reasonable average" predetermined by the court each year, and determine whether the cost is reasonable or not and make any readjustments if necessary. However, the taxing process itself is costly, and the "loser" of this process then bears the costs of the taxing process.
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* If these three points exist, then the other side has a right to ask you to reimburse their legal costs.