The strata plan is the only evidence you can go off.
Two things to look for.
The strata plan will show the stratum ( or levels) of your property. If the garden is a different level to the living area, there will be at least two drawings for you lot.
If the thick line outlines the stairs, then the stairs are OC responsibility. I expect if this is the case thst the stairs are defined in the living area plan.
The second case is that the plan shows the outline of your garden, and there is no outline of the stairs. The plan will have a note to the effect that the lot property extends x meters below ground level, and y meters above ground level. In this case the stairs are owner responsibility.
This situation is one of the trickier cases, and could cause a lot of argument.