There are many different colors and patterns recognized for Wyandottes somewhere in the world (black, blue, buff, buff Columbian, Columbian, golden laced, partridge, silver laced, silver penciled, black breasted red, blue red, lemon blue, barred, brown red, birchen, blue laced red and buff laced).  Some of these are only for bantams, some for both full-sized and bantams, some may be only for full-sized.  I have no idea which are recognized in Australia and I certainly don’t know what your Wyandottes look like.  Knowing that might help out just a bit.
Lavender should be based on Extended Black.  The lavender gene is recessive.  If your rooster is pure for Extended Black, most of those colors and patterns will give you black chicks in the first generation.  Not all, but most.