If you took part in the ACT Champs or 8 hour event there is a good chance you experienced your compass needle swinging around in all directions up on some of the high flat tops. Super challenging when you were trying to find controls like 102 in the dark or mist! This is caused by the underlying rocks, in this case a basalt from volcanic eruptions about 20 million years ago.
Your compass works by having a small amount of magnetite on the needle that aligns with the earth’s magnetic field. However, these basalt rocks have lots of magnetite in them and the effect from these can be stronger locally than the earth’s magnetic field, causing your compass to align to whatever way the magnetite is lying within the rock. This would have been set to align with the earth’s magnetic field at the time the rock was erupted and cooled but subsequent erosion and movement of rocks gives a seemingly random swing of your compass needle.
If you want to find out more about the geological history of Kosciuszko National Park, there is a geological map (low and high res versions) and history explanations you can download from the Geosciences Australia website.
A tip: some backpacks have a magnet on them to clip your water hose on to. Keep your compass well away from this (I recommend taking it off your pack during rogaines) as that too will cause your compass to point in the wrong direction.