I'm not sure that the pipe in the wall ISN'T common property.
However, I would give it one last go and send the owner, agent and tenant a letter telling them that if you can't gain entry by, say seven days, you intend to enter the lot with a plumber accompanied by a strata manager or lawyer to observe that everything is done correctly, and that not only will they have to pay for the plumber but for the locksmith and the strata professional's time too. That might stir them into some sort of action.