One of the few topics on this website I can actually offer advice for as this as in a past life I was a professional fence certifier
Now I was licensed in Qld/NSW but there isn't a lot of difference between states so SA should be very similar. It is very possible the new neighbour is trying for a 'hail mary' and hoping you fall for it. Who did you buy the block from? Was it the developer, a private owner or a builder who did a house & land package etc? The main reason is that will lead into Rob's question above if it was the developer but even then it isn't cut and dry. Often for example developers will offer to assist in paying for fences for the first few houses built in a new estate as it gives the feel of more completed homes helping to sell other blocks neighbouring it. I know this will sound crazy but sometimes developers tell porky pies. Yup shocking I know. It would not be unheard of that covenants might have forced your neighbour to build a boundary fence within a certain time frame. it would not be unheard of that they were told verbally they could chase the future neighbours for 50% during the sales pitch by the one day a week salesperson who sat in an air conditioned donga 7 years ago willing to do almost anything for a sale.
I am not saying the above is even remotely what happened rather what I have seen many times which all boil down to ........ never believe a neighbour chasing money.
Google is your friend mate. A quick search brought up all the info you need to start with
SA.GOV.AU - Fencing and boundary disputes All standard stuff there and will empower you with being well informed as it covers almost all scenarios. If that isn't enough the link at the bottom of that page takes you to the
Fences Act 1975 https://www.legislation.sa.gov.au/LZ/C/A/FENCES ACT 1975/CURRENT/1975.44.UN.PDF Nothing in that document says you have to pay anything under your circumstances let alone 50% of the value from 6 years ago from what I saw at a glance (not a lawyer though).
Just so you know I have consulted on dozens of cases of a neighbour suing for 50% of the price of a fence and not one time have they been successful
after they have built the fence because the legally required notification process wasn't followed let alone 6 years after when you weren't even the owner of the block beside them.
Bear in mind I am definitely no lawyer mate just letting you know what I have seen from my experience consulting to owners being sued and compiling details of the proposed fence, existing fences and neighbourhood standard fencing for them to take to QCAT. I used to get several people like yourself ring me panicking every week because their neighbour knocked on their door claiming they wanted to replace a fence etc and demanding 50%.
Best of luck having Christmas beers with them lol.