The external walls are the responsibility of the Owners Incorporated. The Management Office is usually employed by the Owners Incorporated and tasked by the latter to look after (including maintenance of) the building/buildings. Individual owners should not attempt to tamper with external walls.
I have had a similar problem in two of my rental properties before. In each case, I took photos and attached them to an email to the Management Office. I also called the Management Office and requested they come inspect the problem. They took their own photos. As Green Catalonia says, it takes ages to get the problem fixed. In one of the properties, the Management initially got a "specialist" to inject water-proofing material into the walls from within the property. This worked for about a year before the same problem arose again. They finally decided to tackle the problem from the exterior by erecting scaffolding, exposing the cracks and filling them in before plastering and repainting the exterior wall. This, in my view, is the only thorough method to solve the problem.
These sort of matters usually take a long time and involve a lot of to-ing and fro-ing of emails and phone calls. Bear in mind that the Management Office will have to report the matter to the Owners Incorporated (OI) - usually when they next meet (which can be in a couple of months time) - to see what the OI decide and to ultimately get the green light for funds for any remedial action. They probably also need to invite tenders for the job and this also takes time. You need to convince the Management Office that the leakage is indeed caused by a crack in the outer wall and to bring them on side as to how best tackle the problem (try to convince them that tackling the leakage from outside is a more thorough and longer lasting approach).