Is it something I can fix myself?

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That's a fluidmaster float valve.
Worst case, $40 at Bunnings https://www.bunnings.co.nz/fluidmaster-cistern-valve-bottom-entry_p00268537
Not that hard to take apart and clean anything out that may be jamming it - goes without saying, but make sure the isolating valve/tap is turned off first, or you will get very wet.
Oh, and ping @aredwood if he has a moment
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