I have a retirement visa valid until January 2016. I intend to reapply in Thailand using a combination of savings in a Thai bank and my income. I note an "income crtificate" is required. My question - Is there a specific form? I heard that a statutory declaration from my (Australian) embassy in Bangkok is accepted. If so, is supporting documentation required? I receive an Australian age pension. Any help will be greatly appreciated.

There isn't an official form for the sole purpose of proving an income other than the Stat Dec you get from the Australian embassy. You will need some proof of the source of your pension (updated) e.g Centrelink income statement along with an exchange rate conversion for that day to show the embassy officer and later, the Thai officer dealing with your application. The form you need to extend your visa at the Thai Immigration officer is a TM7.

Do you have a OA visa from an embassy or consulate or an extension of stay based upon retirement you obtained from immigration.

You can not reapply for a visa within the country nor are they extended.

I have seen no reports of the Australian embassy asking for any proof of income when you do a statutory declaration. Whether immigration ask for back up to the statutory declaration depends upon the immigration office you have to go to.