My understanding is that special levies do not require a special resolution, meaning that a simple majority can pass them. Unless there is a serious imbalance in the Unit Entitlements, 5-3 is a majority in this case.
Given that the correct procedures have been followed, and assuming the levies aren’t for some frivolous reason, I wouldn’t think your minority group had much chance at NCAT.
Even if there had been some technical issue (like not sending out agendas) the Tribunal probably wouldn’t change the ruling because the missing three votes would not have made a difference to the outcome.
But then NCAT is a strange beast that sometimes makes odd rulings, so who knows?