I reckon your owners corp has a duty of care to ensure that your parking spaces are available to you. Since they have refused to allow you to install practical bollards, then they need to come up with a workable alternative solution.
My first step would be to write a polite but firm letter saying exactly that, telling them that if they haven’t come up with either a plan to resolve the issue (by a certain date) or permission for you to fix the bollards yourself, you will be pursuing this through Fair Trading under Section 232(2) of the Act, regarding failure to fulfill their statutory duties.
Then, of course, you have to follow through on that, which means applying to Fair Trading for mediation, as a first step. Just make sure it’s more of a hassle for them to do nothing than it is to do something and you’ll get a result.