The Pooyai Ban should be able to witness as well, as they also have an official stamp for when they sign any documents. My pooyai ban ( Mae Luang ) will do for free as she's my wife.
On a slightly different tack, but related, Centrelink shouldn't need to obtain proof of life for expats, especially those in Thailand, and I'm guessing the same protocols would apply elsewhere in the world. When an expat passes here in Thailand a couple of extra steps are required. in addition to obtaining an original death certificate, 3 copies must be obtained. The first copy together with the original is taken to the Amphur (District Office) so they can record the death. The 2nd copy is for Immigration so they can update their records and also notify the home country embassy. The home country embassy notifies the relevant govt departments in the home country including Centrelink if an aussie and the pension is stopped once the last valid payment is made. The 3rd copy is for the bank, so the bank account can be closed and the funds transferred to the beneficiary's account as per the expats last will and testament or in accordance with local laws. Also Centrelink would know there was an issue if the final payment they remit to the expats bank account bounces due to account closure. So that raises a minor question if an expat pensioner passes before turning 80 how does Centrelink know to cease their pension payments? Answer, because of the aforementioned procedure, which makes the proof of life process from 80 onwards a complete waste of time and tax payers money.