As it is common property you would need to get approval from the owners by resolution at a general meeting.
However – I had the same issue – I live in a 1930s block on the top floor and there was no insulation. There were times when it became very uncomfortable for me and my neighbour, to the extent that I got heatstroke on a day when it was 40+ degrees. We put it to the EC that the OC should install insulation as this was in accordance with current BCA requirements. And that is what they did, and the OC paid. They also vacuumed out the roof cavity which was knee-deep in dust, which was probably not very good for one’s health.
We don’t have any sarking in the roof either and that is next on the list.
I don’t think it is a long shot, I think it is the OC’s responsibility.