Because the language is primarily vernacular, you can talk about how the language forms and ratues of Australian slang contribute to the historical value of the text that make it timeless. Because Cloudstreet examines Australia during the post colonial period you can say that it has value both now and in the future as a look back at the development of Australian culture.
Things like the cyclic nature of the text can be used symbolically, and related to timelessness as it can be said to mirror the pattern of actual life - people are born, people die, people are born, people die, and so things just keep continuing on.
The epic or saga-like nature of the text can be compared to real life- wheras most novels have a logical flow of movement and ideas, Cloudstreet is much more lifelike, and examines many different concepts and ideas in no particular order, mirroring actual life.
The discussion of themes would obviously be important, bu make sure you talk about values such as family and place, and give a discussion of how it reflects the values of the society in which it was written, and challenges the values of the society in which WE live, as family values are diminishing.
I hope that helps
