Missing works, retaining wall

Hi there,

Just need some advice.

I am just over 3 month post handover and I have only just realized that the builder did not build the retaining wall as specified in the drawing plans.

I have also checked the building permit and it says one of the 'structures' is a retaining wall.

Obviously since this is post handover, the surveyor that issued the occupancy permit had signed off on it missing the retaining wall.

Need some advice on what to do, should I just let the builder know and are they required to fix it.

I understand that I signed the PCI and it is post handover so will they put it back on me saying I signed all the works has been done?

Are they compelled to finish the retaining wall since its on the plan and that I have paid for.