Mel prawns are a tough one IMO. All prawns are frozen out at sea when they are caught, they are then brought back to land & left as is (whole green prawns) or are shelled whilst still frozen. Otherwise they are thawed & cooked (some shelled some left) then frozen again. The process is very safe from a listeria point of view. After that is when it becomes blurry. When you go to the fish markets, supermarket or fish shop & see prawns that are "fresh" don't touch them, they are 98% of the time time thawed & not really fresh. It would be rare to get actual fresh prawns unless you go to a prawning farm. If you purchase the prawns frozen, and I mean HARD FROZEN, then get them home & keep the in the fridge to defrost they will be fine. Make sure you eat them when they are still very cold & don't let them warm up, so if you want to have say 20 prawns in a sitting, I'd take half out & shell them / eat them then the other half out just to be sure.

If you like PM me & I can give you a website with a list of distributors in your area so you can get them in a proper frozen state. I work for a food wholesaler & we get them direct from the manufacturers so I have some contacts in other states that do the same thing as we do.