What is the point of fining them. By the time the police turn up, hours later, they could all be possible infected, issued a fine and sent home.....to go on to infect others in the community. How about a little common sense.
There is talk of entrants into the country being put into hotels for ten days into quarantine at their expense. How about doing the same to theses party goers at their expense. There would be the financial penalty, paying the hotel (and guards) plus protecting the community through quarantine.
Or that just too simple an idea. Non payment would be the same as with any non payment of fine, prison. Drastic, yes, but potentially infecting others is totally unacceptable too.