A new firmware update with build number 23.1.A.0.740 has been certified by the PTCRB for the Xperia Z3 (D6603), Z3 Compact (D5803), Z2 (D6503) and Z2 Tablet (SGP521). This compares to the latest Lollipop build on these devices with build version 23.1.A.0.726. We’ll let you know when it starts to roll.
Thanks MikePL!