Just thinking ahead a bit here.

Went to get a re-entry permit and the immigration officer commented that my passport was full and I would need to get a new one soon. I have to go back for a 12 month extension in January. The passport has 3 more years validity.

On examination the first three pages after the observations page have not been used, likewise the next two pages after the middle page with the chip, I have one free page at the end.

1st question is these vacant pages can still be used right so really I am right for another year or so depending on how many re-entry permits I get? (I only go to Australia and they don't stamp so there is no issue that end).

2nd question is, my Cambodian paper visas from a few years ago are starting to fall out, even though you can still see the edge of the stamp where the visa has been, could these pages be used provided of course the paper visa's were kept with the passport?

1 - Yes the normal pages can be used, assuming they are normal pages and not those set aside for observations. I had a similar issue where the Immigration Officers just stamped the first page they came across, my passport actually had plenty of space for stamps, it's just they were scattered throughout the passport, I just pointed to a suitable space and the IO obliged, I've actually find the Thai officers to be quite amenable, unlike the Australian officers who seem to stamp a passport in such a way as to almost waste a whole page - that's my experience anyway.

2 - If you're talking about an actual visa, then no I wouldn't be removing the visa, you're setting yourself up for problems.

In your position, I would probably be getting a new passport now to save yourself any problems, I renewed mine last year, it had over three years left.

Thanks for that. As far as renewing goes how does your existing visa go from the old passport to the new? I am inclined to wait until I get my 12 month extension in January (February it comes through) and then renew it. Is there any foreseeable advantage in renewing it in Australia rather than at the Embassy, as I'll probably be in Australia March.

You do not transfer visas, but you can transfer an extension of stay, you just go to your immigration and they will do the change for you, some officers insist on a covering letter from your embassy.

I really don't know whether it's best to change in Thailand or Australia, it really makes no difference, it just depends which is easier and more convenient fro you.

If you do it in Australia, make sure you retain the passport and that the page with the extension and re-entry permit isn't damaged, and then show the old and new passports when entering Thailand.