You mentioned receiving the Oz gov't. pension. I assume you mean the Oz Old Age Pension (OAP), which by Oz legislation is exempt from Aust. personal taxation.
As is the Oz DVA Service Pension and/or the Oz Permanent Disability Compensation Allowance, previously called the DVA Disability Pension, all exempt from Oz personal taxation.
Receiving any of these payments probably means any other income is quite small (but I quickly add, that's not my business to assume).
My Guess (just my guess, nothing more) is that Thailand will not 'classify' any of the above payments as taxable in Thailand regardless of 183 days residency etc. And in any case the total amounts would probably mean that person is under the lower income threshhold.