Postally used, although why it was necessary I cannot imagine. 6d and 1 shilling bisected is 9 pence. If they really had run out of stamps they could have used a 6d and a 3d or any other combination so I strongly suspect that there is an element of philatelic usage in this.
Still an interesting piece of philatelic history and it may be listed by Pemberton in his article (6d.green. and 1/- (S.G. 171) both bisected, although this one is S.G 172 lilac.).
The provenance is the "Colonel Harrie Evans" Colonies and Pacific Islands collection auctioned by Prestige Philately on 4th May 2002. Auction 96, Lot 400. The auction catalogue can be found here. 3 items were for sale in this lot, with the auction description being, "1882-91 Type '11' Perf 12 6d green & 1/- violet bisects fully tied to separate pieces by Brisbane duplex of DE6/93 or FE23/94, plus both bisected (!) on piece with indistinct duplex. (3 items)."