Teaching kids sign language is a great way to boost their vocabulary!  There have been tons of studies showing that learning a second language when growing up can boost a child's intelligence and help them learn as they grow older.  I don't have any children of my own yet, but I do know that the first signs I learned from a friend were awkward and turtle, which came into a surprising amount of use in our conversations.

I wonder if there are any baby signing classes in New Zealand you could take your child to.  I know here in California there are SignShine classes not too far from where I live, but I don't know what options you may have near you.  Classes like that would be great for helping you and your child sign together