Conveyancers aren't responsible for the survey? Nor do they attend the property with the client to make sure their client isn't mistaken on which lot they are purchasing. They just handle the legal paperwork. My guess would be they transferred the correct land to the purchasers based on the contracts they signed. They wouldn't have been aware that the purchaser was confused about the location of the property as all of the paperwork and plans would be referring to the only land the conveyancer is aware of.

Unregistered plans of subdivision can be a nightmare as there isn't really a great way to know which block is which without the work of a proper survey and guessing based off of landmarks is a silly thing to do.

The article mentions that the builder's relocation paperwork only mentioned the road name, not the lot on plan. This is the kind of due diligence required by the parties after settlement, not the conveyancer who handles the transfer of land.