The bank's shares plunged on Thursday after S&P cut the bank's credit rating on worries about its property lending.
Figures in the Federal Gazette showed short sellers, whose bets speculate on a fall in a company's share price, held positions against 8.08% of the bank's shares, up from 7.57% earlier in the week.
The figure may underreport the positions because only bets of 0.5% and above are required to be publicly disclosed.
PBB declined to comment.
(Reporting by Tom SimsEditing by Miranda Murray)