Apple wireless keyboard seems to be stuck on volume increase button and others...

I don't even know where to start with this question, it's really random. I got home yesterday and woke my computer from sleep mode and I had a browser screen up that had lots of jumbled crazy text in the address bar and the page was max zoomed in as though a cat had sat on the keyboard (which is not the case here).

Then suddenly it started continuously making the volume increase sound, and was at max volume but just kept making the sound anyway and whenever I tried to drag the slider back down it would just increase back up again and none of the keyboard key would work. It was also zooming in and out on the browser page and typing in more endless text and characters into the address bar like crazy. So I turned off the keyboard and it all stopped. Restarted the computer with fresh batteries in the keyboard - same thing happened again. Gave it a bit of an upside down tap incase something was in there, but still happening.

So I've just plugged in a spare logitech keyboard (which I hate typing on after being used to the nice flat slim keys!) and it's working fine so obviously it's the keyboard that's the issue. Just wondering if anyone has had the same with a resolution?

Using a late 2009 27" iMac, not that that matters!