Is it your roof? or is it common property (which would be usual)?
The owners corp has the right to enter either common property or lot property to carry out repair work to common property or even to see if repairs are requires.
In the former instance, provided they aren’t going through lot property, that right is pretty much unlimited.
In the latter case, if the lot owner or residnet refuses access, the OC can go to NCAT and get orders.
I they have started work on a common property roof without prior notice, that is just rude and inconsiderate, but it’s not illegal.