Not quite true. FP 13 was built for passenger work, the others were built as pay busses.
OK, let's put a bit of fact in here :
FP1 to FP6 were built by Waddingtons in 1937 - in fact it was Waddingtons FIRST railway contract - two were built as single-ended and the other four as double-ended - originally built as passenger railbuses, they proved to be unsuccessful in that role, and within two years five railbuses were converted to paybuses (no mention of what happened to the sixth one !). In December 1941, FP5 was blown up and destroyed in an armed holdup killing all crew on board. Waddingtons got an order to build a replacement for FP5. Source : 'History of Comeng' by the late John Dunn - Vol 1 pages 40 and 45.
FP7 to FP12 were built by Comeng in 1968 - built solely as paybuses to replace the aging FP1-6 series - all were double-ended and air-conditioned. Source : 'History of Comeng' by the late John Dunn - Vol 3 page 36.
FP13 was built by Comeng in 1969 as an 18 passenger EXPERIMENTAL railbus - it was intended for use on the Cooma-Bombala line to replace 600/700 class diesel railcars which were over-capacity for the prevailing traffic. A number of significant deficiencies showed up, and the experiment was deemed unsuccessful (no mention of what happened to the unit after withdrawal). 'History of Comeng' by the late John Dunn - Vol 3 pages 37 and 38.