The USDA plant website says the Shellbark Hickory (Carya laciniosa) is hardy down to -22C, and the Shagbark Hickory (Carya ovata) to -44C. These are generally regarded as the best nut producing Hickory trees. (Assuming that's what you want them for).
St. Lawrence's Tree Nursery has some very good cold hardy hickories although I don't know how you'd get them shipped to Australia but its worth to check it out.
-18 degrees C equals just a bit below zero F, which means USDA zone 7. Which means you can grow any hickory you want, pecans would be my personal choice but I like pignut and shagbark. I also think I have hybrids of the two growing in the woods but no seeds. Again, pecans would be my choice, and IMHO seedling trees are better; candy companies over refuse to buy named variety pecans, instead preferring seedlings, the oil content and taste is stronger so less has to be used in the candy.
I second the Shell bark (Carya laciniosa) and Shag bark (Carya Ovata) for cold hardy eating nuts. I have encountered both in Central Minnesota pretty far into the Zone 4. Pig nut Hickories (Carya Glabra) are a tasty, with a smaller nut, tree I have seen do very well in Zone 6. Black Walnut (Juglans Nigra) and Butternut Walnut (Juglans Cinerea) are both tasty cold hardy nut trees in my experience too. Butternut gets a blight that might not be an issue outside of the US.
Location: Deepwater northern New South wales Australia
posted 7 years ago
all i need now is a mule to get them here??