You need to see the soil profile, is it fill over natural clay of is it natural topsoil over natural clay?
Your neighbour may be discharging stormwater onto the ground or there may be a overflowing pool/dam.Finding the source
of the excess water would be a good idea and maybe the best option and not cost you anything depending on how you get on with your
neighbour.
If that doesn't work then a cut off drain near the fence line may be the best option especially if
you can't hook up to your stormwater system.
Dig out the soil profile then you can work out how deep you need to go with the cut off drain if you can't find the source.
I agree that a cutoff drain would be better than a spoon drain near our neighbour’s fence line.
What is the difference between fill versus natural topsoil over natural clay? A neighbour told us that when the area was just vacant land, they added piles of topsoil over it, so my guess is that it’s fill over natural clay. How can we tell?
Our neighbour doesn’t appear to be discharging stormwater onto the ground. He has water tanks close to our fence line, and his overflow seems to go somewhere underground (not sure where though). He also doesn’t have an overflowing dam. When it rains a lot here, there is a lot of surface water. Even our neighbour’s land holds various pools of water. He has built his land up higher than ours though, so any surface water falls into our property.
When you say “dig out the soil profile” - what exactly do you mean? I’m happy to dig, but I’m not sure what I’m looking for. I guess it would be obvious when I hit clay.
Look for the clay depth if you can take a photo of the pit you dig and post it I will have a look, also take some photos of the animal paddock. See if there is water seeping over the clay layer when you dig it up. What area are you in ?