If you are coming from the south and have visited the fjords or any of the signposted waterfalls in either of the passes, then you may not be impressed with Dorothy Falls.
If, like me, you're a waterfall-whore and enjoy swimming and bathing in nature, then this may be a nice detour on the way to/from Hokitika Gorge. Waterfall is a 5-10min drive from Hans Bay on an unsealed road. Do-able for a car/van/small camper, but not advisable for large,rented, 2wd motorhome. If you're sleeping at Hans Bay, you can walk the road early morning/early evening. Across the road from the falls is a short walk to the lake - well worth a look. In total ~3hr return walk from Hans Bay. Or if you're driving through, total stop-off time ~20min. Add on time for swimming - both the waterfall and lake are good options and you can often have the place to yourself.