Interestingly, it has 3 fiscal cancellations. There is an obvious large box Sandakan D11 type cancel with 4 circular bars rather than 3 and within it an violet oval cancellation of the judicial department of Sandakan. At the bottom, there is a faint green postal like cancellation with part of the word Sandakan. The colour is rather unusual and unlikely to be postal.
These stamps are probably fiscally cancelled with postal markings. The $5 has the large box circle cancel D11 but does look suspicious. The $10 has the Sandakan BNBC postmark D9(apparently very rare) which is normally used for fiscal purposes.
Update I agree with the comment regarding the $5. Note the CTO bars at the right top corner. NB is full of fakes. I am still paying my "school fees".
Judicial Department Sandakan BNB type 2
From these two stamps, one could see two similar Stamp Office frankings, one with larger letters. These latter day high fiscal values are probably quite common.