Looking at how it seems layered along the top I'd say it's sandstone, but well "aged". Is it grainy? In time sandstone develops a growth of lichen which gives it that dark colour. A scratch would reveal the colour below. The area from around Newcastle down to about Nowra is all sandstone and it's fairly readily available from landscape suppliers.

Thanks Tropicbreezent, but it's not sandstone. I've identified it now, it's known as "bush rock" and it's very difficult to get flat blocks of it.

Hate to state the obvious, but "bush rock" is sandstone, particularly in the area where you are. The thing with calling it bush rock is that it's weathered and already has a growth of lichen (and/or moss) on it, the way it would be found out bush.

Tropicbreezent, thanks for trying to help, but as you say, to state the obvious, sandstone is yellow and has a grainy texture when broken. I should know, I've dug down into it in some areas of the garden. Bush rock is completely different. A quick search online produced this definition: "Bush rock is a pale colored igneous rock, it consists of large individual grains, visible to the naked eye. The common components that make up bush rock are quartz, feldspar and small amounts of mica, amphiboles and other minerals. This combination tends to give bush rock a gray, red, or pink color, with dark grains speckled throughout the rock."

What you're defining there is granite. Being pedantic, the term bush rock means any rock found in the bush. Rock that is quarried and cut to shape isn't referred to as bush rock. The quarried rock has a fresh surface whereas the bush rock has a patina which takes a long time to develop at the surface of the ground. It's that patina that makes it desirable as many people want the 'natural look'. Hence using that term to define that characteristic. But it's in limited supply because it's illegal to just go out and collect it.

The thing with granite is that when it weathers it flakes. The flakes are mainly curved and leave the original rock generally rounded, such as you see with the Devil's Marbles in the NT. To get flat granite pieces it needs to be artificially cut, so definitely not bush rock. Sandstone fractures in layers and occurs like that in the bush. It's very variable, depending on the particles binding the sand grains together. So it can be soft or hard, and the colour varies also. It's the sandstone which is the bush rock around Sydney.

These are quotes from 2 websites advertising bush rock in the Sydney area:

"Our ever popular sandstone range includes bush
rock, flagging, sawn stone, dimension stone, seawall blocks, split blocks and
garden edging and ..........."

"Sandstone Bush Rock is a timeless, popular
product that can be used in a variety of ways, including retaining walls and
feature rocks."