If they assured you by email I would print that reassurrance email and take it to the interview. I have read of people getting denied even with Uni degrees because they did not have high school certificate. However, it probably depends on the country. The Sydney consulate may be more lenient than say a consulate in a 3rd world country, where no high school certificate may raise some questions. I had my year 12 certificate, year 10 and degree certificates - but Sydney consulate only wanted to see the year 12 certificate.
I will print that email for sure, just to be safe.
Although I think it makes sense. The official requirement says you must have completed high school or a course to get into college. That's what they use as the bar. So if you have advanced college degrees then obviously you were admitted to college and must have completed or satisfied the requirement to do so.
I find it hard to believe anyone from a first world country with a higher education would be refused on that basis. Do you have any more info on the people refused with uni degrees and without highschool degrees? Probably they were refused for some other reason....