@Carsten ***********
I suggested something like this sometime ago and was howeled down. Your suggestion is a very wise one.
Notwithstanding, should your only income be your Australian aged pension there is no need to Lodge and Aussie tax return. However, while there is no obligation to Lodge a tax return it would be wise to lodge one showing the pension on said tax return as is suggested in this post. This way, the Aussie pension is shown on the tax return, attracts no Aussie and therefore hopefully should be tax exempt on the Thai side.
However, the above is based on retaining Australian tax residency. I'm uncertain as to what happens or how it works if you lose Australian residency status