Found basically in the middle of bushland in suburban Brisbane. To get here requires walking through overrun dirt paths that pretty clearly haven't been visited in a long while.
My wild ass guess would be that it's a gauge for measuring water levels, with the upper opening part being a scale to read a marker that moves up and down the pole.
Does it line up with anything on this map of Brisbane water/sewer infrastructure? That map seems likely to give you at least an idea of what system it's associated with, and might even give more of a clue about the purpose of this specific point.