Yes, tampering with the passport is illegal. Post-it notes for obvious reasons not, as you say. If you alter the information in your passport, then you do something wrong.
If the stamp on your Cambodian visa is not 100% on the sticker but also partly on the page, it will be even more apparent. This guy here got caught with it.
After all however, it seems that the country that put the visa there might have the biggest issues with you removing it. I would think that they know your passport number and that you got a visa before. If you ever want to go back to the country you will have a big issue at immigration when the visa is missing.
Have you tried to call the Australian embassy and ask for advice? It takes only 10 days to get a new passport in Bangkok, and if you pay more, you can get one with 64 pages. Having only 3 pages left is a good reason to get a new one. I do that all the time, I have a stack of old passports at home.