I've seen an RFDS PC-12 pass from left to right underneath a QF 737 I've been on (as a passenger) on the way into Adelaide.
Must have been at least 2000-3000 feet below, but even a PC-12 can look relatively large at that distance.
May well have been me. Will certainly never forget it. Centre "warned" me of the crossing traffic but if I remember correctly it was only 1000' as I'd stopped climb at FL180 and it was descending to and had just reached FL190. Let's just say that if I had not had the heads up from ATC I may have needed a change of
underwear!