There is plenty of vacant land in Auckland.
In terms of construction, we advise you to find flat land, without a long driveway, and with drainage (both stormwater and wastewater) if possible. Also check that the land doesn’t have any drainage issues, floodplains, or a significant amount of trees.
Some Auckland areas may have solid rock under the ground as well. We’d recommend checking a geotechnical report if it’s available.