How to replace seating washer on Hardie Bathroom products cistern

[KieranTyson]

Hello,
I am in Australia and I have noticed a leak from my cistern into my toilet bowl.
Upon investigation I have surmised that this is due to a failing washer. The washer appears to be disintegrating as there are many small black pieces of rubber in the cistern and it flakes easily when touched.
I have a Hardie Bathroom Products cistern attached to a fowler toilet bowl.

I can see where the washer is but I cannot seem to get access to it. I cannot seem to remove the piece which connects to the flush mechanism.
The attached photos show the piece I am unable to remove.

Any advice on how to remove this piece so that I can get access to the washer and replace it is appreciated.

Thanks in advance.

 

[moonlight]

The most likely reason water is running in to the pan is the seal on the bottom of picture 5. This can be purchased separately. Good luck

 

[rpm]

It is normally the bigger black washer at the bottom of the flush valve that degrades.
Sometimes it is easier to replace the valve leaving the part in the bottom of the cistern in place. Blocks placed in cisterns to colour the water and clean attack rubber.