Some 250,00 years ago there was a forest area at the Northern end of Takapuna beach when the sea level was far lower. Rangitoto island erupted, and molten lava surrounded the larger trees and as it solidified formed deep cones. The timber continued to burn as the molten lava cooled forming a fossil forest that is still visible particularly at low tide.
Just recently probably due to some recent storms and rough seas gooey sandstone has appeared in a greater amount than I can remember observed as the tide recedes about halfway along the kilometer long Takapuna beach. Sticking out of this mix of sand and mud are many pieces of blackened wood appearing like small tree stumps.
These are probably branches and trees that have floated out from rivers and have been captured by the sand and mud. I will try and find out more about this strange phenomenon.