During day my pups have had supervised time with chickens. At night pups can be out without supervision, but make certain chickens are roosting in dog proof locations. During night is when the pups start to become protectors, but I do not want much more than barking at threats until dogs are pushing a year. You can ruin a dog that tangles with a tough customer like a raccoon too early. When the dog barks, get up promptly to see what is up. Dog will figure out it can recruit you to help and you can egg dog on when it is after what you want chased off or dispatched.
Soon we will have a litter of pups that will get exposure to a lot of free-range chickens. They will only be able to access adult birds and they will be supervised at first. Once pups get big enough to run chickens down, supervision will get more intense as that is when real training through intervention begins.
Aussies are high energy and I assume you chose that breed to do a job, maybe to compete in agility, be a jogging partner, herding other animals etc. Without a job, he'll create his own... like herding the chickens or the neighbors' animals.
I wouldn't try to keep them apart too long, your pup will know there's something super interesting out there and the fascination and anticipation will only build the longer you try to keep them a secret. The goal is to socialize and eliminate that mystery. Even though it sounds like your pup will already be somewhat familiar, you still might want to start fresh.
I trained all my dogs the "Leave It" command and it's been a very valuable tool throughout their lives, past and present. Start practicing "leave it" in a quiet distraction-free environment working in 5 or 10 minute increments a couple-few times a day, for several days. Use many different boring to mildly interesting items to be sure he has a reasonable understanding of what it means before moving out to an area with chickens in view. Give very high value treats (like tiny pieces of hot dog or roasted chicken) as a reward every time he looks away or acts uninterested.
Of course you can walk him outside among the chickens on leash without using the command if he hasn't learned it yet. You want to set him up for success every time you do use the actual command, so just walking around the birds and other surroundings for regular socialization, and praising for making eye contact with you is great.
Currently I have two dogs, a yellow lab and a blue-nose pit bull. Our lab was already grown and trained when we first got chicks so she already knew how to behave, but our new pittie was alarmingly interested as a pup. I worked with her using the "leave it" command and offered her yummy treats only if she broke her intense gaze and walked away, and we played games (on leash) in the presence of loose chickens in the yard.
Dogs are smart and know when you unclip the leash, so keep the chickens penned for the first off-leash outing near the birds and test the command.
Keep your pup busy with other off-leash training lessons or playing until you're confident there won't be any undesirable chasing. If it does happen, use the command and hopefully he'll come right back to you. Reward highly for that good behavior with a super special treat or lavish praise. If he does continue chasing, scold and return to more leash training. Most dogs (and puppies) want to please their owners, and as long as you're consistent and you also provide an exercise outlet (boredom causes mischief), chickens will eventually be nothing but the background. And don't worry, you won't have to keep treating forever, you just want to set a strong "code of conduct" from the very beginning.
My dog Daisy totally ignores the birds now because like many other dogs, she discovered her own high value reward... cleaning up all the chicken poop
Snappers are very good at catching critters in the water, and that will include small swimming birds. No swimming chickens here!
I've noticed that the wild geese on our pond don't seem to have losses that way; good care on their part, and lack of caution with domestic birds? Those Canada geese keep the wild ducks out of our pond too, sadly.
Mary