Caroma Caravelle 270 shut-off valve doesn't

I have a Caroma Caravelle 270 with the dual flush, which is about 3 years old now. I live in an area with extremely hard water and I suspect that a lime build-up may be keeping the shutoff valve from closing completely, allowing water to leak into the bowl all the time. I knew how to fix the old toilets with floats, but I don't know the first thing about where to look to get this working. Does anyone have a link with information on how to fix or replace the valve in this toilet? Mine is similar to this one:
https://terrylove.com/forums/attachment.php?attachmentid=12697&stc=1&d=1301794382



Thanks for any advice or suggestions.

Karen

Karen, if it is the shut off valve, the extra water will be going down the overflow pipe into the bowl. If that is not the case, then the flapper is likely leaking. Flappers are easier and cheaper to replace than valves. Might be a good idea to check this out before tearing into or replacing the shutoff valve.

I will definitely check that first. Thanks for the reminder.

Nope, it's the shutoff valve. When I manually close it after a flush, water still runs through, and when it reaches the fill line, water continues to flow in and into the overflow pipe eventually. How do I go about fixing that?