Welcome to the Bunnings Workshop community. It's marvellous to have you join us, and thank you for sharing your question about getting a fence extension for your Colorbond fence.
I take it you've seen this discussion - How to add an extension to a Colorbond fence Project by @ProjectPete. The parts for the Colorbond fence need to be ordered at the special orders desk. But before you go, I suggest taking a few photos of the fence and measuring the height and width of one section of your fence from post to post.