Welcome to the Bunnings Workshop community @TessaJones36. It's sensational to have you join us, and many thanks for your question about enclosing a verandah.
Before you start, you'll need to check with your local council to see if a permit is required. Apart from that, I don't see any reason why you couldn't turn this into an enclosed room. It has all the makings of one, including a roof, the structural posts supporting it and a floor. You have excellent high beams under the roof, which will be perfect for insulation. However, what you'll likely end up with is a room that's tacked onto the house. It's probably never going to be quite as air-tight and insulated as the rest of the home. This somewhat comes down to your ability to build the walls and install adequate insulation.