As interesting as such a loco would be, I'm going to call BS on this one. I don't see why Graincorp would have any use for a 165t loco that's heavily restricted in use, especially a new build of a 20 year old design (Even if PN had 4 built 10 years after the originals, I don't think many people would still build them nowadays).
Knowing how nosy enthusiasts are, such a loco would have been photographed at least a dozen times by now and there'd be rumours floating around yet there's nothing to be heard anywhere until now about it.
Not to mention the 05-1696 serial number puts it in with the 2005 order from PN and overlaps the 1696 with the builders number of now scrapped BHP SD70ACe #4300
Though I'd love to be proven wrong, it'd make for something very
unique to go out and catch crawling along at 40 odd K's an hour with a string of 48's behind it