Te Henga(Bethells Beach) to Muriwai is a nice day walk that I do fairly often. It starts on Bethells Road just after you have crossed the bridge and then crosses over the Waitakere River.
The Waitakere River isn't so big now. Most of the water is send to Auckland. As you travel out to Bethells there is the Te Henga wetland to the right. This swamp wasn't so large when the full river flowed but now is one of the most important wetlands in the Auckland region.
On the other side of the bridge there is a tap for water and a gate to keep livestock in and then you walk towards the sea gaining height along the way. It isn't hard and it has great views across the beach and back inland to Lake Wainamu.
Near the top there is a good resting spot looking down on the lagoon and the beach. People have carved their names into the sandstone. If you know the area they are a bit of a who's who of the children. Below there, near the lagoon is tapu though so please avoid it.
At the top of the peninsula there are some views down into Ihumoana Island and Waitakere rock which the whole region is named after. Originally the river turned right near the sea and went around the island and past Waitakere rock but it changed its course after the dam.
The trail then goes down to the old kumara fields and O'Neills Bay. Just before you see the sea along the kumara fields there are the old pits for cooking.
Further on a small spring flows across the trail. The water is good although it tastes of the irons that runs through the sandstone above. The trail then runs in behind the sand dunes before rising steeply up to the end of O'Neills.
That is pretty much the end of any hills until you reach Muriwai. The trail winds around the edge of the cliffs with fantastic views down to the crashing waves. At the other end is a series of steps that take you up onto the ridge. After that, it is all downhill to Muriwai. Or, you can continue to Goldies bush.