The process is to apply for mediation, which is an obligatory prerequisite before you take them to the Tribunal seeking orders under section 132.2 compelling them to fulfil their obligations.
Often the very fact that you are determined to take the next step may be enough to galvanise these recalcitrants.
You’ll find a mediation application form HERE.
By the way, the logic in this case is quite simple. The OC raised the level of the floor to undertake work that was for everyone’s benefit. You have paid your share of having that work done (even though it seems it is of little direct benefit to you).
To make the work compliant with building regs, they have had to raise one section of the balustrade. That means their work has made your outlook uneven and ugly. Their legal and moral obligation is to fix it.
By the way, if they choose to fight this at NCAT, you can ask that all costs be awarded against the OC in a way that means you don’t have to pay a share of the special levy that they will be required to raise.