I think you would need to contact some local builders who will come round and look at the wall and advise you on what step to take next. Also what a lovely drive way! Love the purple flowers!

Thanks Catty, I have since swept up the fallen blossoms! I am on a budget but think I might actually need an engineer. There was a landslide further up the hill resulting in a neighbour losing his front lawn. The task of removing and replanting some large hibiscus shrubs further up the drive is what I am really feeling worried about. It would be a rather daunting task to save and replant all the shrubs. We alternate between droughts and bushfire danger, to torrential rain here!

Wow, that's alternating weather! I know it could be expensive but i do think it is the best way to go to avoid future issues. If it did ever come to replanting the shrubs just keep in mind that it is for the best and maybe get some help from family and friends?

A landscape architectural company should be able to advise you.

Thanks for your helpful answers. A problem shared is a problem halved, so they say. Goodnight. It's late down here...lol

It sounds like the wall is under stress from the weight behind it. This will increase if water can't drain away so one solution may just be to make some drainage holes through by drilling or removing the mortar around some stones.
Do you have insurance? It may be worth speaking to your insurers first as they may have approved contractors who can advise you.