The simplest way to respond is to ask the other owner to allow access for you and you meter readers. If they refuse or delay, explain that they have done work on common property without proper permission and they could be liable to undo the work they’ve done at their own expense.
It’s hard to be too specific without know what parts of this garage or door are Common Property and I would keep the threat up your sleeve until such times as they prove to be unreasonable but if you need to, this is the relevant section of the Act:
116 Owners, occupiers and other persons not to interfere with structure of lot or services to lot
(1) An owner, mortgagee or covenant chargee in possession (whether in person or not), lessee or occupier of a lot must not do anything or permit anything to be done on or in relation to that lot so that:
(a) any support or shelter provided by that lot for another lot or common property is interfered with, or
(b) the passage or provision of water, sewage, drainage, gas, electricity, garbage, artificially heated or cooled air, heating oil and other services (including telephone, radio and television services) through or by means of any pipes, wires, cables or ducts for the time being in the lot is interfered with.
(2) The owner of a lot must not alter the structure of the lot without giving to the owners corporation, not later than 14 days before commencement of the alteration, a written notice describing the proposed alteration.