My three day excursion of finding Lulumahu Falls took place some time ago, and all though there are more hints on the internet now than there was then, there is still really no good directions from what I can find. There is however a few web sites that I have come across that have blaitant lies on them for the main purpose of dissuading and keeping others off of trails like these. This trail may or may not be trespassing. I truly do not know the answer to this, but I do know this, the board of water supply does not use this property any more and has not done so for a very long time and from my trips back into this area and from all my friends who have gone, not one person I know has ever been hassled, ticketed or arrested. I truly do not believe that anyone has ever been cited or arrested for hiking back in this area, but that is my opinion, although I believe that I am probably dead on with this assumption. Certain web sites and blogs tell people not to go to places like these because, in my opinion, these same people want to keep these trails to themselves or only take those that they deem worthy. The situation is laughable but do not let posts from otheres fool you and dissuade you from exploring this area (luakaha, kaniakapupu ruins, Lulumahu Falls, Mo'ole valley, etc, etc). All the falls in this area are absolutely beautiful and if everyone in the hiking community goes to these places whenever they want, with no worries (and in huge groups at times), then there obviously is not very much of a risk of getting in trouble, otherwise everyone would not be doing it.
The nice thing about Lulumahu Falls is that it always seems to be flowing, even when other falls, like Luakaha nearby are dry, making it one of the few waterfall hikes you can pretty much do year round. As with most hikes, please treat these places with respect and don't act a fool while on the trail and at the falls or ruins, and please do not climb on the ruins like an idiot (they are in bad enough shape as it is).
The best way to get to the trailhead for Lulumahu falls or Kaniakapupu ruins is to take the pali highway (coming from Waikiki) and act as though you are heading for the Nuu'anu Pali lookout. Just before getting to the pali Lookout there is gonna be a street on your right hand side that you want to take, named Nuu'anu Pali drive. If you follow Nuu'anu Pali drive all the way to the end it will eventually reconnect with the pali highway. Just as Nuu'anu Pali drive is about to reconnect with the pali highway there will be a open turnout spot just to the right with some spots for cars to park. Once parked there, there will be a gate directly behind with a hunter check in station and a gap in the fence. Go through the gap in the fence and take an almost immediate right and head towards the gravel road (you can also go to the huge gate a few yards down and crawl through the hole on the left side, but people always seem to be there in my experiences). Follow this gravel road until there is a trail going left with bamboo on both sides. Take that left and follow the wide open trail. The trail will turn sharply to the right but will stay wide open. When the trail ends at a bunch of trees look for a small opening and a trail that goes through briefly through the woods. If you follow this trail it will bring you out to a clearing. At some point make your way up the hill. There are stairs but if you don't see them just trailblaze your way up the overgrown vegetatioin shortly and you will end up on the board of water supplies property (which they don't use and noone is ever there) and reservior four with another gravel road. Follow this gravel road until just before it ends and head up some concrete stairs on the right side and there will be a wide open trail to follow. Just remember, if ever in doubt, just head upstream. The trail jogs from one side to the other once on the stream but as long as you stay near the stream and look for ribbons you should be good. As for Kaniakapupu Ruins, park in the same spot and walk back down the street on nuu'anu pali drive for about five minutes (be careful and watch out for oncoming cars). You will eventually see a board of water supply tank on the right side of the road. Just across the street from the board of water supply area there will be a faint but somewhat obvious path on the left side of the road that you will need to take. Just after starting on this trail you will begin walking through a bamboo forest and will take a left onto a obvious trail shortly after. This trail will take you directly to the ruins. This trek should take no more than ten to fifteen minutes.