We received another downpour yesterday, which overflowed the tank again. Only this time we had installed a temporary tyre retaining wall.
It was a freely available resource we could aquire at any tyre fitting business for nothing. Best of all it doesn't damage the tank being butted up so close. The size of tyre which seemed most popular was the 175's. Other sizes didn't seem to come up in multiple numbers beyond 2 or 4.
There's actually two tyres to every tier here, but you only see the one sticking out. It took us the best part of a day to ram them full of dirt so they wouldn't move. They did the job too!
The water which escapes from the inlet pipes once the tank is full, now diverts past the tyres instead of cutting a deeper channel beside the water tank. Our main concern was the stability of the base of the tank.
It's not going to be the permanent solution but it will certainly do for now.