What's this?

I'm no plumber but I would say drainage/storm water. I doubt sewer would be vented to ground level and a sniff would probably answer that anyway.

What is on the other side of the wall? is it a bathroom or kitchen? It could possibly be your overflow relief gully or a disconnector gully that was once charged by nearby fixture discharge pipes. On the other hand it could be a stormwater drain, but being outside a paved area with no apparent grade towards the grate it seems unlikely. Especially this close to the building.

On the other side is our dining room. There is a toilet/laundry to the left and a kitchen to the right. There may have been a toilet where the laundry is a long time ago, but I can't be sure. It looks fairly deep, about 500mm. I can't smell any odour.

mmm a WC wouldn't be run to a disconnector gully, the lack of odour is probably due to it being full of dirt, without further inspection I would expect it to be an old part of your sewerage system but anything is really possible

Just looks like plain old stormwater to me. That style is every where up here on the older houses - clay pipes and metal grates

Is it at the lowest point around the perimeter of your house? if so could be a Gully,
Is there a garden tap above it, if so could be a Gully,
Is there a gully somewhere else on your property?, if so could be stormwater then,

If the area around it was once paved then its most likely stormwater.

Run a hose down it, see if it comes out on the street??
Put your hand down it??, or a hose down it....blow out the water & sniff for sewer smells


Ive got no idea....but plumbing is interesting isnt it, I shoulda been a plumber
A real plumber will tell us, hopefully.

Remove the metal grate, run the hose down the drain, locate your boundary trap, assuming you have one, [ a property service plan is available from your water board, to establish if you are in a b.t. area or not]. If water from your hose is running into your boundary shaft, it is definitley sewer. If so run fixtures one at a time and check if they flow into the shaft like previously with the hose, if yes it is probably a dodgy disconnector trap, that may need repair by a Lic. plumber. Noticing that area has been re-cladded with aluminium, it is possible that this was originally a 4" sewer vent that may have been removed when cladding was installed and not restored. A site visit would take a plumber 5 minutes to establish.

If that is a gully it should be above ground level I'm guessing ?