Knob Coral Dipsastraea maxima
I am pretty sure is Dipsastraea, the sp is more tentative butlooks it can be the one.
Corallites large and round, oval or polygonal. Septa regular, thickened at the wall and with well-developed paliform lobes lobes forming a crown around the colunella. Brown or yellow brown with pale green oral discs. (description from Corals of Australia and The Indo-Pacific by J.E.N. Veron, page 463).