Toilet back-flow – whose problem?

We recently renovated in our just bought 60s apartment in Sydney's eastern suburbs. Upgrading the bathroom we replaced the old, leaky toilet with a new one. The new toilet never seems to flush through properly and is subject to continual backflow, which becomes foul when we are away for a few days. We have had two plumbers look at the problem, our own and strata manager's. Our plumber says the problem is due to the branch stacks behind the bathroom wall not being angled high enough for the new toilet, a case of the new standard toilets not marrying up with the old plumbing. The strata plumber told our strata managers that the problem was due to an incorrect install, something the strata plumber supposedly denied when our plumber contacted him to discuss it. Either way, I am confident it is not an incorrect install and that it is a branch stack problem. (The stacks have been blasted recently). Apparently it has occurred in a number of other units which have changed over to modern toilets. The strata managers insist this is our problem and refuse to get a third plumber to inspect it as suggested by the dept fair trading. Does anyone out there have experience or knowledge of what the strata law is when a lot owner's new, regulation toilet doesn't work with old plumbing behind the (strata) wall?