It sounds like columnaris or a similar bacterial infection. You may want to isolate her in a quarentine tank if you can and treat her with antibiotics. I'm not sure what's available to you in Australia, hopefully someone like Angry_Platy will be along and can recommend something. I like to use a broad spectrum antibiotic like kanamycin or nitrofurazan. I don't know if the Jungle brand Fungus Clear fizz tabs are available there but if they are they work well as long as you treat for 8-10 days. I've treated pregnant swordtails with both of those antibiotics and it did not seem to have any harmful effects on the fry. You want to act quickly though as columnaris can kill them very quickly. Is she a recent addition to your tank? Do you have any other swordtails or other livebearers in the tank? You'll want to keep a close eye on them as well. Check their mouth area and also their fins, look for traces of white fuzziness on the fins, this is a pretty common place for it to first show up.
