First of all, if you like to hike to the falls; don't leave any valuables in the car! Car break-ins happened recently. The falls are located in the northern part of the Waitakere Ranges.
Unfortunately, parts of the Waitakere Ranges got closed due to the Kauri Dieback disease in May 2018. Also, the trail to the Fairy Falls is closed at the moment. Check out which trails in the Waitakere Ranges are open and which ones are closed. PDF Hiking Trails Waitakere More information about the disease, and the status of closures for hikes and campgrounds can be found here.
Update closure August 2019: The Fairy Falls Track remains closed.
If you travel to the famous Piha Beach, don't miss this marvelous rainforest and the Fairy Falls. The falls are most impressive after heavy rainfall, but hiking is much for fun in the drier season with pleasant temps from November until April. Important; if you like to hike to the falls don't leave any valuables in the car!
Warmest and driest months are during summer from December to March with pleasant temps. Coldest months in winter from June to August. Most precipitation during winter but rain already starts in May with its peak in July. Anyhow, moderate to significant rainfall can always occur.
It is a popular trail and easily accessible. Anyhow if you have any mobility issues, it might be too steep to get to the falls. Especially during summer; the peak season and at weekends more people walk this trail. Nevertheless, this hike is still not overrun. Less busy in March, April and November with cooler temps but still lovely.