Those pyramids at Stockton sand dunes are tank traps put in place in WW2.
Some info here on them http://www.thelongpaddock.net/stockton-beach.html
There are some under the main beachfront as well
https://www.theherald.com.au/story/3082408/heavy-seas-reveal-tank-traps/
Thanks! That's good information. Learn something new everyday! (actually I feel kind of silly now as these things actually have such a serious use). Thanks for pointing it out!