A beautiful place to visit, although a few important things to note that you don't easily find online;

Limited parking, very busy spot but plenty more parking down the road at the beach 10mins walk back, (so you can drop off your car load at the Lake Wainamu car park before driving down there).
Parking Wardens do monitor the area so if you park where there is no parking signs you may get ticketed.

Limited toilets, toilets are at the beach, and one up on the dunes just before the lake (can be difficult to find though).

Black sand can be extremely hot, bring suitable footwear and definitely not suitable for prams of any sort.

Dog walking areas at beach (Lake Wainamu is a dog free regional park).

Sliding down the dunes seems to be advertised on blog posts a lot, although what they don't tell you is the bad accidents that happen to some people that do this. The westpac helicopter has been out several times this Summer.

No petrol stations or food shops, there is just lovely cafe but it open at the weekends and is at the beach.

Lake Wainamu is beautiful but most information sites don’t give the full picture of what facilities are there, which is essential none, so you need to be really well prepared to fully enjoy the day here. First of all, dogs are not permitted at the lake. There’s very limited parking at the lake car park (which is still 15 minutes walk from the lake). It’s often full by early/mid morning on weekends after which you need to park at the beach 1 km away and walk back. The beach is also where the main toilets are and also the only drinking water supply. There’s no mobile coverage so you need a plan in advance if you’re meeting someone there. There’s no permanent shops or petrol. The fabulous Bethells cafe sets up down at the beach carpark but currently only operates on weekends and can be weather dependent. Trip Advisor will point you to others but they’re at Piha - a 37 km drive away. That said, it’s great, but maybe not everyone’s idea of perfect.

The carpark to the lake is quite small but you can park closer to Bethells beach as there are more spaces there.
To get to the lake you need to go through the gain dunes first. We opted to climb the dunes at the start so that we could walk around it at the end of the walk. After walking up the dunes, just keep walking in that general direction and you should eventually arrive at the lake. The lake is quite large and does look quite nice. There were quite a few people swimming in it but be wary that the shore of the lake drops quite suddenly. There isn't much cover on the lake so it would be wise to bring some sort of shade to protect you from the sun if you're planning to stay long.
After arriving at the lake we went to the trail that goes around the lake. It was quite scenic with nice views of the lake as well as the dunes. There is a larger area on the other side of the lake which heads to the waterfalls.
The falls are multilayered and you are able to hop into the pools to cool down a bit. We went further up the falls and the pools closer to the top had less people. We were around the third or fourth level of the falls. It looked like you were able to head further upstream but we opted to stop there, admiring the views. You can either back track or keep going along the lake and you eventually reach the pier for the lake. Instead of heading back up we just went around the dunes, back to the carpark.
It was about a 2 hour walk to get around the lake. It seemed that most people just stopped at the lake shore and stayed around there. It was an enjoyable walk and it was fairly easy, the hardest part being walking up the giant dunes.