Hi
The https://tools.geofabrik.de/osmi/?view=areas&lon=152.94037&lat=-27.54332&zoom=19 multipolygon cannot have outer role parts that share sides.
(the semicircular part is sharing sides at the end of the main building)
Easiest fix is to delete the overlapping or shared sections.

You could also map using building:parts which calls for the entire perimeter tagged as building=* and all the individual parts tagged as building:part
https://wiki.openstreetmap.org/wiki/Key:building:part