Not sure what the price is in Australia, but in the US the Lee holder is only about $15 more expensive than the Z-Pro...not a big difference considering the advantages to the Lee setup. The Lee wide angle adapter ring is a bit on the pricey side, however, with the Lee holder and WA adapter you won't get any vignetting on a FF camera, even at 16mm with all three slots in place. Can't say that about the Z-Pro...most folks have to do workarounds with the holder in order to eliminate vignetting.
As far as the filters go...yes, any 4-inch wide filter (100mm) will fit in either holder, with possibly one exception...the Lee Big Stopper. The Z-Pro uses three small plastic pins that attach the holder to the adapter ring. The pins protrude into the space below the first filter slot, which basically prevents one from using the first slot (closest to the lens) without a bit of modification. The Big Stopper has foam pads on one side of the filter...with the Stopper in the first slot, these pads sit flush against the face of the holder and prevent light leakage. OTOH, the Stopper is a 4x4 filter...its possible that it may work in the Z-Pro's first slot, but can't say for sure. 4x6 filters are an issue in the first slot, however. Not sure how folks do this with the Z-Pro because of the protruding pins...you may want to check into this before making an investment.