How do you join the flues, do they go inside or outside of each other going up.
Welcome to the Bunnings Workshop community @Brian20. It's wonderful to have you join us, and many thanks for your question about flue installation.
Typically when adding the next section of flue pipe, it will slip over the one below. This creates an overlapping joint between the two sections of flue, which helps to prevent any smoke or gas leakage. However, it's important for you to refer to the manufacturer's installation guidelines. Please note your flue might need to be installed by a qualified person whose work conforms with local council regulations, Australian/New Zealand standards (AS/NZS 2918:2001) & manufacturers' recommendations. You might also need the installation certified for home insurance purposes.
Please let me know if you have any questions.
Mitchell