As you can imagine, building a satellite was not something we had any experience with and nor had anyone in Australia in the 1960s. Nevertheless, there was plenty of knowledge overseas and some written down in books. Remember, there was no internet then, so conventional libraries held the books in which that knowledge was available. Not having any space experience, we wanted to assure ourselves that AO5 would not only survive but also operate as we intended once in orbit, and so we undertook testing of components and the completed satellite.
We simulated the temperatures that we expected with fridges and ovens. However, we also wanted to check components in a space environment and we knew we could access "near space" with balloons with weather balloons and large scientific balloons. High altitude tests on balloons had the advantage that the balloon could be seen by a large number of amateurs and this became a way of enthusing them about the project.
So, once the designers and builders of the individual components were satisfied with their products, they were tested in "near space' on high altitude balloons and in chambers on the ground.