It should be so easy!
There are heaps of useful YouTube videos showing how to take the cistern apart, remove the flush assembly and even how to replace the washer and get it to bed evenly and fully into the the little seating groove around the center of the flush assembly.
While many express the frustration of getting that new washer to successfully seat so that it actually DOES stop the piddling toilet, none of them tell you what to do when all their instructions fail!
I've asked plumbing specialists and even they say it can be cheaper to replace the whole cistern than the cost of the time they waste getting these things to stop leaking!
We have two different models of Caroma toilets in our home, they were both fully replaced when we reno'd the house in 2005.
They are NOT the "better" range of toilets but rather the "dime-a-dozen" all-plastic variety mass produced under the Caroma brand.
Nonetheless I would have hoped that "servicing" them would have been less bothersome than it has proven.
As of today (Jan 2017) I've had to replace the cistern flush-washer three times in each cistern.
That doesn't seem so bad if it were as easy as taking the assembly out, ripping the old washer off and stretching the new one back on ... oh that it were THAT easy!
EVERY, and I mean EVERY! time I've done this, I have battled for ages to get that new washer to seat properly so that it doesn't continue to slightly leak after replacement.
So the last TWO times I have discovered that if you CANNOT get that new washer to seal properly on it's own, both times I have been able to stop the trickle by first ensuring the new washer IS perfectly evenly sealed into the inner groove of the flush assembly and THEN ...
... THE TRICK ...
Using the old washer, (or another new one if you prefer) mark out a larger internal circle as shown
(I used the larger end of an old bayonet light fitting ferrule to mark the circle with a paint pen)
Fold the washer as shown
and start the cut using a pair of sharp wire cutters or heavy duty scissors
Cut around the marked circle to remove the inner portion, then stretch the remaining outer portion over the newly installed washer so that it sits snugly underneath the new washer between the washer and the flush assembly, to offer a more level supporting platform for the new washer to press against.
As I say, this has worked for me BOTH the last two times when EVERYTHING else has failed to stem that final trickle.
It may still take a few moments or a couple of flushes to bed everything in, but it was successful for me.
Please add a comment if you want more detail or even just to let me know if it worked for you too.