It would be an issue if it passed the "checkout date" on your TM30 receipt when you apply for anything at the local immigration office.

You just update your TM.30. With checkout date filled with longer period or it's open. Some put the date of lease expiry for checkout date field.

Q. Is it possible to do this all online or do I need to visit an Immigration Office?

-> You have to go to the office in person. There is an eExtension service, and you can apply with an extra fee added to the official fee, even though, you have to go to the office in person to get a sticker on your passport on appointment date with eExtension service.

Q. which Immigration Office would you recommend?

-> It's more like depending on from where you go to the office.

Q. can I apply 2 weeks in advance?

-> Yes. The Chiang Mai office accepts extension applications within 45 days of the current stay expiry.

Q. can I make photocopies of passport and take passport photos at the Inmigration Offices?

-> There are copy shops around offices.

@Paul *****

, Unless you travel somewhere after that and you have a receipt, you don't need to re-submit.

It wouldn't be a problem. Just update your TM30 before you go to the immigration office.

It's supposed to be submitted within 24 hours after you arrive at that address, not 24 hrs from entering Thailand.

Not submitting TM.30 violates regulations, but submitting multiple times does not.

There are people who re-submit TM.30 to get a receipt just before going to the immigration office.

No, Australians don't need a visa to stay in Thailand for tourism purposes in the short term.

Visa-exempt entries are still in place but it's 60 days now.

I think you are mixing eVisa and ETA. Check this post, please.

5/

@Ryan *******

, more and more embassies state you need to be in the country until the visa issued, or application could be rejected.

As you are still in the same country to travel to Thailand from Vietnam while you wait for approval , it wouldn't be a problem.

There is a well-written User's Guide for the TM.30 web system, in English and Thai.

Go to the link at the bottom of the login window of TM.30 website. Check the guide well including FAQ.

@Andy ********

, That's correct, you have to apply for an extension for the same reason as your entry/initial stamp. You can change the reason for a 1-year extension when you already on 1-year extension.

Yes, it's possible if you do carefully. As you say, you need documents from two branches for next application.

BUT,

better to talk with your local immigration office before you move money. Some offices don't prefer those steps. Some don't have issue for you to do that way.

Make sure you will be prepared in advance.