Extension science is more like an introduction to university research. It helps to think of it as "extension investigating science", although you don't need to take investigating to take extension.
Course Content
Module 1 looks at the history of the scientific method, including the philosophical origins, what constitutes "science", error, bias, etc. Basically, its module 1 (Thinking Scientifically) of year 7 science, but at a year 12 level.
Module 2 is your research proposal. It's mostly to do with formulating a hypothesis, gaining a better understanding of the field you plan to explore, and so on.
Module 3 is looking at data collection and analysis. The inquiry question explains it best:
How is evidence used to make decisions in the scientific research process?
Module 4 is your actual research report. Usually, you'll spend the second half of term 2, and all of term 3 (until submission) doing module 4, because it's weighted as the most significant portion of your marks.
Assessments and Course Requirements
There are a total of 3 internal assessment tasks for the course (plus the HSC), and it must be taken with at least 1 2-unit science (Physics, Chemistry, Biology, Investigating, Earth/Enviro).
Usually, Assessment Task 1 is presenting your research proposal, and includes elements of module 1 and 2 (but mostly mod 2).
Assessment Task 2 is module 1+3 and, depending on your school/teacher, is the only written paper for the whole course.
Assessment Task 3 is your major project. There's no trial paper for this course (unless your teacher doesn't like you), so your major is weighed 40% (may vary depending on your school).
Major Project
The extension science major project is a research report into a scientific field of your choice. You are expected to conduct research, analyse data, and produce a "functional" scientific report (like you would find on JSTOR). The report should be 2500-3000 words, and include diagrams, graphs, and whatever else is necessary to present the findings. Universities such as UNSW and USyd also run programs in the 6-week holidays to give you essentially a crash course in Science Extension. For more information, take a look at the Science Extension Student Guide (I can't post links, but just Google it).
HSC "Paper"
The HSC "Paper" for extension science is (at time of writing) the only digital HSC exam (hence the quotes around "paper"). It takes 1.5 hours, and is usually one of the last "papers" in the HSC block.