75 tier points is the correct amount to be credited for a long haul sector. So if you travelled LHR-SIN and then SIN-SYD you would receive 2 x 75 points = 150. However, if you did a straight through LHR-SYD on the same flight/ticket then you would receive 135 points (this is the only exception to the 75 points per sector rule, due to the length of the journey).

So it sounds like you have been given the correct amount of tier points.