Is it legal to peel off a visa from your Australian passport?

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.

It is 'technically' illegal in general.

The Australian Passport Office has:

Damaged passports

It is your responsibility to keep your passport intact and in good condition. Normal wear and tear will not affect its usability, but serious damage to your passport could prevent you from travelling overseas.

Contact with water or other liquids can cause serious damage. You must not tear or remove pages from your passport. It is critical that all the details and the photos on the personal data pages are legible and clear, and that there is no evidence of alteration or tampering with any aspect of the booklet.

If you are unsure whether your passport's condition is good enough for travel, you should seek advice from the Australian Passport Information Service on 131 232 or from an Australian diplomatic mission or consulate. You may need to take your passport to an Australian Passport Office for assessment.

Under the law of most countries, passports are government property. A visa that is a wet ink stamp can't change that and a sticker format is still a visa. In the rare cases where governments take back passports they have issued the visa stickers are not first removed by the government and handed back to the person to whom they were issued.

So, a passport with a stuck in visa can't be considered to remain intact once an attached visa is removed, with or without evidence of the removal.

I am not sure about Australian passports but mine, when new, only had one page of personal data. Australia mentions all the details and the photos on the personal data pages are legible and clear. I do now have personal details (name, date of birth, photographs) throughout my passport, mostly as part of self-adhesive visa stickers.

Finally, there is no evidence of alteration or tampering with any aspect of the booklet. It is not specified that any evidence must be visible to the naked eye. (However, in many cases removal of my stickers would leave evidence such as incomplete wet ink stamps, parts of signatures or other ink marks.) With modern forensic techniques it is extremely unlikely that removal (of adhesive or stapled) visas would be impossible to detect.

Clearly many people have managed to remove visas without serious consequences, though several have faced extended questioning and reprimands. But then again, many people have exceeded speed limits without facing any consequences, even though against the law.

The confusion may be that removal is a crime without an effective penalty. USA for example has a law about Destruction of Government Property but the upper limit of the sanctions imposable a fine of up to $250,000, ten years imprisonment, or both would be out of proportion and a more reasonable withdrawal of the tampered passport hardly worth the bother.

For USA there would be no point:

Do not try to remove the visa from your old passport and stick it into the new valid passport. If you do so, your visa will no longer be valid.

I saw a man cry when his brand new passport was confiscated at the airport of his own country because a page was a bit chipped at a corner. He looked like he was on route to a foreign job and leaving country for the first time ever. Imagine the devastating experience.

Don't tamper with your passport in any way. Don't remove stickers. Don't fold or tear pages. Don't wet it. Don't peel off deliberately any plastic or other material even if it's naturally peeling. Don't write anything in it, even to fix or make legible something that was already written by an immigration officer. Don't try to erase anything written or stamped by authorities. All of this could land you in trouble for illegally modifying your passport.

If you ran out of pages, just get a new passport at the nearest embassy. They will cancel your current passport and issue a new one. Your existing visas will still be valid, so just bring both passports to use them.

If you really need to have a new visa added on your current passport and you don't have a page for it, request the issuing embassy for an alternative travel document given your situation.

Don't worry about exit stamps or entry stamps too much. As long as there's enough space somewhere in your passport to stamp it without crossing over other stamps or stickers, the checkpoint officers will let you through - especially if you're going home. You don't really need a clean box specifically for the stamp.

Post-it notes are okay to attach and remove because they are specifically designed to be able to remove from any surface without trace or damage. Also, they are not government property (as opposed to passports or visas) hence removing them is not illegal.

However I would still recommend you not to apply post-it notes inside your passport. It can be a nuisance to the immigration officer and the last thing you want to do at the airport is to irritate him or her. What I normally do is to find the relevant visa page and keep it ready while waiting in line. I give my passport with the visa page open and visible. Some officers seemed to appreciate this as they can quickly stamp me in, but others ignore it and flip through the entire passport anyway.

I also don't jacket my passport because you have to remove the jacket before presenting the passport to the officer. Normal wear and tear doesn't invalidate your passport, so you only need to take good care of your passport and you don't need to buy a jacket for it.