I’m going to take a punt here and say that becasue the stairs were built on common property with the clear knowledge of the Owners Corp at that time, they ARE common property unless there’s a document that says they aren’t.
However, I am not a lawyer and I haven’t seen the plans; this is a serious issue and it’s time you have a chat to a strata lawyer about your options. I would think they would include:
A clear and definitive assessment of what is and what isn’t common property.
A letter to the members of your EC warning them to stop the harassment or they will be served with an AVO.
If the lawyers believe the stairs are common property you would then want a letter to the EC warning them that if they continue to resist paying for the repair of the stairs and someone is injured in an accident, they may not only be liable for damages but could individually face criminal charges under the new Work Health Safety laws.
Go HERE for contact details for Makinson & D’Apice, send them an email, tell them you raised the issue on Flat Chat and ask them what they think (and how much this is likely to cost)