Some Australian bachelors degrees are more than 3 years. Three is generally the minimum (unless expedited by summer courses and overloading), but a Bachelor of Liberal Studies is four years, as are some that qualify people for professional practice in an area. Some are 5 or even 6 years. Further, in order to get Honours, it's generally necessary to get a high average mark in the first three years and then do an extra year of study. So, for example, a Bachelor of Arts degree is usually 3 years, and a Bachelor of Arts (Honours) is four years.
No, a US Bachelors is not equivalent to an Australian Masters. Generally, the Australian Bachelors degree is the first degree in a subject, or a generalist degree (like BA or BSc) that might precede something like Medicine or Law. An Australian Masters degree is a higher degree in a subject, after the basics have been learned in the Bachelors degree. However, something like an MBA might not require a first degree in business, but just a relevant first degree, so it depends a little on your subject area. I believe this is similar in the US.
US exchange students from bachelors programs at top schools like Harvard and Berkeley who attend good Australian universities tend to be no more advanced, and sometimes less advanced, than good students doing the equivalent bachelors courses at good Australian universities. The bachelors courses of top schools in both places seem to be of a similar quality. [probably, on average, the Australian Year 12 high school graduate has a broader, deeper education than the average US high school graduate, but of course there are good and bad schools in both places. Sadly, I fear the Australian standard is slipping as the schools become obsessed with standardised tests in place of thinking and learning]
Someone else said Australian Masters degrees are not that useful in Australia and are just for foreign students. In my experience, that's simply not true. A masters degree from one of the good Australian universities can make someone hugely more employable. In some areas (such as law and biological sciences), your career is likely to be hindered by not having further qualifications beyond a bachelors degree. Australian Masters degrees can be extremely rigorous, and most of my Masters students are local Australian students who are wanting to improve their qualifications and be more highly regarded. They must write 8,000 word papers (or equivalent) for each subject, and the exams are at least as difficult as those for equivalent courses in the US.
HOWEVER, there has been press coverage of degrees, especially at some of the less prestigious universities, that were basically created so that education could be sold to foreign students. These were particularly tied to the old immigration system that allowed people to apply for Permanent Residency if they'd studied certain subjects in Australia. This discredited system has been revamped in the last 12 months. I understand that this has led to a drop in foreign enrolments in such courses, which is leading to some of those courses being phased out. Thank goodness for this. In my opinion, those dud-degrees did a lot to damage the reputation of Australian tertiary education, which is very unfair when the rigour of Australian degrees from good universities generally compare favourably to those of good degrees just about anywhere else.