You are correct in relation to the next steps. You will need a surveyor and a strata lawyer.
Before you start spending funds, a strata lawyer will need to know:
–When you bought your townhouse
-And have a copy of your contract for the purchase.
The strata plan should set out the boundary of the lot property in a heavy line. Anything outside that heavy line that is not marked as lot property (that is the unit you and others own) is therefore common property. The lawyer can engage a surveyor to survey the various boundaries between you and your neighbours . That survey will then be used to compare with the registered strata plan that you were provided by the vendor (who is often the developer) when you bought your townhouse.
You may also need to consider the value of the land that you apparently anticipated being part of your lot, before the developer decided that he needed it more that you did! You need to be informed about the value of the land taken before you spend money to reclaim the land. You may well have a case for redress, but hasten cautiously and with the right support.
We are happy to advise further.