In WA the retaining wall and fence come under different Acs. The retaining wall comes under the Building Act and the fence comes under the Dividing Fence Act.
Under the Dividing Fence Act both property owners are jointly responsible for the cost of a standard fence being stratco in WA.
Under the Building Act it is apportioned according to which property has changed the lay of the land.
This is the same for QLD.
I doubt very much the plans at the council will provide the information you are looking for to determine where the lay of the land has changed especially with it being over 30 years old.
The best option is to hire a surveyor who will drill a couple of bore holes in each property and they can determine from the layering of soil and other methods where the land has been changed. Depending on the landscape around the property another method is to use the lay of the land elsewhere as a baseline such as the road. There are survey pegs which measure the height above sea level and these can be used to determine the fall over say 12 meters and then measuring the fall over the two properties across the same distance. Does it follow the same as the road and if not in which property has it changed? This too requires a surveyors site plan to be completed.
We are having this issue with our neighbours. The whole cost, for 42 metres of fence, is $6,000 retaining wall and $3,000 for the stratco fence. Neighbours want to go half however the previous owners of their property changed the lay of the land (dug out when building their patio) and the old fence is now leaning into their property. I don't feel like giving them $3,000 towards the cost of the retaining wall as we weren't responsible for the change in the lay of the land. I suggested, in the fairness and best interest of all parties, we get in a surveyor to give us a report. Surveyor's bill to be split by both properties.