A repair assembly in the form of a temporary mold for repairing holes in plasterboard sheet walls comprises a backing plate for location within the cavity wall formed by the plasterboard sheet against the rear surface of the sheet over the hole to be repaired, a front cover for contacting the front surface of the sheet to close the hole, and a connector for connecting the rear plate and front cover to each other to form the mold, having a mold cavity further defined by the edge of the hole. When the repair assembly is clamped in place by the connector, suitable repair material, such as uncured plaster, is injected into the mold cavity through either the front cover or connector, to fill the hole. After the repair material has set and/or hardened, the connector and front cover are removed leaving a repair region flush with the front surface so that little or no extra finishing of the repair is required apart from filling in the small depression formed by the connector, thereby saving time.