Charcoal chicken has been a favourite takeaway of Sydneysiders since home-delivered pizza was considered a hot new trend and Kingswood Country was primetime viewing. Buy a chook, choose a few sides, feed a whole family – the more roast potatoes the better.

Chef Josh Niland and his wife Julie are now applying the same chargrilled takeaway concept to fish with a new store serving family packs of juicy, golden-skinned Murray cod.

"We flog charcoal chicken at home, largely because we have three kids to feed," says Niland, owner of acclaimed Paddington restaurant Saint Peter and its retail arm Fish Butchery.

"It's one of those experiences that doesn't feel like health food, but it's not junk food either. It's just a nice dinner option that takes the fuss out of things.

"There are great fish-and-chippers around Sydney, but we want to take the concept further … we want more people to see fish as a really simple, day-to-day luxury to engage with."

The Nilands' much-anticipated Charcoal Fish eatery opened this week on New South Head Road in Rose Bay, not too far from Chargrill Charlie's chook shop.

With the exception of a yellowfin tuna double cheeseburger, the menu is built completely around white-fleshed Murray cod – Australia's largest freshwater fish.

"The reason why it's pretty much all Murray cod is because I can cook the whole menu from one single fish," says Niland. "To me, that's sustainability in a nutshell."

Fish heads and bones are used to make gravy; wings (the meaty part of a fish attached to its pectoral fins) are grilled and spiked with a tamarind hot sauce; fat is rendered to roast potatoes.

"When the fat makes up such a significant amount of the Murray cod – and we've had some fish with 8 per cent fat in extreme cases – that comes at a financial cost to the business," says Niland. "If that fat goes in the bin, we're already on the back foot."

Murray cod's healthy fat also helps protect the fish from becoming dry. "The fillets are also processed by us in a way to make the skin go like pork crackling, or the best crispy-skin chicken you've ever had," says Niland.

A $60 half-fillet pack with pickles, baps and bronzed chips will feed three to four people. Rotating salad and side options include kohlrabi remoulade, barbecued peppers, and radicchio with sweet-and-sour grapes.

Customers can expect to part with $22 for a rotisserie Murray cod roll loaded with deeply savoury stuffing and crisp skin for texture. Lip-sticking fish bone gravy brings the sandwich together.

Fish fat-roasted potatoes have a stronger taste of cod, but the $20 double-pattie tuna cheeseburger – made with leftover yellowfin from Fish Butchery – could almost pass for its beef-based cousin.

Fish fat ice-cream is on the cards, but Niland needs to see how much fat he can yield from the half-tonne of Murray cod he has ordered.

"If we get 25 kilograms of fat, then yep, absolutely we're putting ice-creams on the menu. But if it's eight kilos, the fat will likely just be used to roast potatoes for now."

Aquna Sustainable Murray Cod, based in the Riverina, will supply the shop with all the fish it needs.

"I approached Aquna and said hey 'I want to open this shop, but I need to know you're not going to run out of fish in six months' time'," says Niland. "They said, 'absolutely we won't'."

Largely due to natural habitat destruction and overfishing, commercial catch of wild Murray cod has been banned since the mid-1990s. Only farmed Murray cod can be sold commercially and Aquna's focus on sustainability made the business an appealing choice for Niland.

"We're in the middle of the Murray-Darling Basin out here, so it's the perfect environment for farming the premium fish," says Aquna chairman Ross Anderson.

"We also release a couple of million fingerlings back into the river system every year to help reintroduce the species."

The romance of cooking wild-caught fish in season is still there for Niland, and Saint Peter will continue to showcase Australia's best King George whiting, tuna and coral trout when the restaurant reopens after lockdown.

"I do find myself letting guests down when someone asks for King George whiting at the restaurant though, and I have to say 'I'm really sorry, but [whiting season] doesn't start for another month'," says Niland.

"One of the best things about this Murray cod is that people can rely on its comfort all year round. Hopefully it can get a new generation of kids eating great fish too."

670 New South Head Road, Rose Bay, charcoalfish.com; Open Wed-Sun noon-3pm and 5pm-8.30pm.