You need an anti-helminth medication. In most parts of the world, this will require a trip to your local vet.
Historically Levamisole, Piperazine and Praziquantel were often recommended, but they don't work at all reliably, and Fenbendazole and Flubendazole are now recommended as being much more reliable. Some aquarists have used dog worming tablets, but this is a risky approach and I wouldn't recommend it. There are aquarium-specific medications available, for example in the UK "Fluke M Koi Treatment" or "Discus Wormer Plus", but you may or may not have access to these in Australia. Whichever medication you use, remember to follow the instructions, dose correctly, and remove carbon (if used) from your filter.
Yes, Camallanus will cause health to decline over time, and yes, Camallanus worms can infect other fish in your aquarium. This needs to be treated, promptly.
For what it's worth, Camallanus worms are particularly common among certain types of farmed fish, angelfish and livebearers in particular seem to be very widely infected by them. You may want to complain to your retailer, as this is something retailers can do much to prevent. Properly quarantining new livestock and sourcing from good rather than cheap fish farms are both critical issues.
Cheers, Neale