Mackennal bust identical on all Australian bronze and silver coins 1911-36.
8 pearls on the crown band but picture not clear enough to examine wear on the band of the orb** on top of the crown.
Nice luster is obvious.
At least AU-53, most probably AU-55, but could be AU-58, if no wear as above **
You will need to grade that feature**, hand held, under 10x magnification, for yourself, just as the third Party graders do. Same applies to the high points of the leaves on the reverse.
With those sorts ways, you should be able to nail the grade down accurately, and no need to have it slabbed.