It's a "secret code" that's in every street in the Illawarra - and it could save your house from burning down.
On power poles - or streetlights in those neighbourhoods with underground electricity lines - there hangs white metal plates with the letters H and P and R and sometimes a few small numbers in between.
For many, they might go unnoticed but for the city's firefighters, they contain vital information to fight a blaze.
For those who have always scratched their heads, the tags are markers showing where a hydrant is located.
A sign with H and P means the hydrant is in the pathway. An R means it's on the road.
A line between the two letters means it can be found across the road, while numbers can indicate the distance from the plate to the hydrant.
There are also what's known as secondary markers - a green H nailed to a post.
Sometimes it might even be a blue reflective marker in the middle of the road.
Fire and Rescue NSW Inspector Chad Wallace said locating a hydrant is vital as a fire truck carries on average around 2000 litres of water.
"If you're fighting a normal house fire, one hose uses about 230 litres a minute," Insp Wallace said.
"If it's a bigger house fire, you'll now have two hoses and you're now using 460 litres a minute.
"So you've got about five minutes until you run out of water.
"That's when your crew is then looking for markings on telegraph poles to try and get a hydrant.
"It can affect how you fight a fire. If you know you can't get that water quick, then you can't use as much water."
With improvements in technology fire crews also carry a laptop that can provide the exact location of every hydrant in a street before they get there.
Which is handy, because people have been known to park on top of them. Or even turf over them.
"If people really like their garden they don't like the plate with the concrete. That's when we dig them up," Insp Wallace said.
"The computer tells us we've got one in that front yard so we'll measure it out.
"If it's not there, we get a shovel out and we start digging and nine times out of 10 we'll find it."