You are referring to AS/NZS 4600 - 2005, yes the equation is incomplete. If you look back in AS/NZS 4600 - 1996: Section 3.6.2, equ. 3.6.2(1)... Mb=1.25*fy*Zf, where Zf is the full unreduced section modulus for the extreme compression fibre.
Equ (2) and (3) similar.