you cannot get Visa extenmsions with 40k income, you'll need 400k as Australia does not do the income affidavit and most Immigration Offices do not accept 40k transfers. You can transfer your 400k back after you get your extension if you wish or use it for living expenses.
Get a 90 day Non-O before travelling which you can extend in country with a "Visit Thai Family" 60 day extension giving you 5 months or a 12 month extension and a re-entry permit which gives you your 6 month stay plus your entry the following year (this requires 400k in a Thai bank account for just over 2 months each year at the time of applying for the extension)