In the ACT, for Class A units, which means apartments, any wall lot boundary lies “lies along the centre of the wall”.
That’s defined in the Unit Titles Act 2001, Division 2.2 Boundaries.
A door is an element of a wall.
When I’ve been in a similar situation, with an apartment in the ACT, it was in the end a 50-50 cost split between myself and the OC.