Nine days ago we took our research intern out to the west coast. In a change from our usual Waitakere destinations, we went directly to Karekare beach, walked south through the dunes into Whatipu (passing the tramline tunnel), inland at the Paraha valley, and then back to Karekare through the hills along the Zion Hill track. It was excellent, although the Zion Hill track was quite muddy. The whole walk took about four hours.
It's probably a little better to do the loop in the other direction, to get the up-and-down stuff out of the way first. We did the beach first because I hadn't been that way before and maps warn that it might not be passable at high tide. But either those warnings only apply during a big storm, or the dunes have grown since those warnings were written --- it didn't look likely that a high tide would cut off the path. So next time I'd definitely take the Zion Hill track first if the weather's reasonable.