It is insect damage and it is worth trying to figure out what is causing it.
Don't use systemic, especially if you have pets. That is not leafminer damage or any kind of borer. Leafminer damage looks like brown or white squiggly lines on the leaf. A stem borer would cause wilting. I don't know about Australia but leafminer is not a known pest of coleus around here.
It is probably a caterpillar. Sometimes the caterpillar take on the color of the leaves they are eating and it makes them that much harder to see. Look for fras (caterpillar poop) and then look on the stem above that. That can give you a clue.
If you don't find any caterpillars and since you mention you keep your plant outside, it could also be a beetle that flies on and off the plant. You may never see it. Try looking at the plant at night with a flashlight. Around here we have Asiatic Garden Beetles and June bugs (types of scarab) that fly at night and eat things like coleus. It wasn't until I started examining the plants at night on a regular basis that I found out what has been destroying my plants all these years. Those I just have a small bowl of soapy water, hold it beneath the plant and knock them in. Spray won't work well on that kind of pest because you have to hit the bug.
If you suspect slugs, look under the pot and see if they are there. You are on a balcony so you should be able to tell if you have them or not by looking beneath nearby most locations.
