Hi @Monica2883,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Unfortunately, due to the height of the retaining wall, you will likely be required to gain council approval which would likely require an engineer to produce plans which are then submitted to council for approval. They will be able to design a suitable method for anchoring your retaining wall as well as help you select materials suitable within their design.
This is because a number of variables, including the composition of the soil, hydrostatic pressure, drainage, and potential for damage to neighbouring properties will need to be considered.
Let me know if you have any further questions.
Jacob
