Speaking from the perspective of someone who has a PhD from Australia, the difference is that Australia / New Zealand base their programs around the UK model. Second, to get into a PhD program you have to have finished a BA Honours degree which is a 4 year rather than the normal 3 year undergraduate degree (not sure how long a US undergrad degree is mind you, maybe yours is 4 years anyway, I don’t know). Also it used to be that generally PhD candidates would have completed an MA first. And in fact that was the main factor really- you would do a 3 year undergrad degree, then an MA and only then would you be approved for a PhD. So that is 5 years before you start your PhD. Recently MA programs have been mostly scrapped as a pre req for PhDs in this neck of the woods so you only have that extended undergrad qualification (personally I think BA Hons do NOT prepare students for PhD experiences but others might disagree).

The other thing is that the PhD program is itself fairly loosely constructed in the antipodes. You will apply to enter a PhD program but first have to get approval from a prospective supervisor. You will work under 1 primary supervisor and maybe 1-2 other secondaries. In your application as a PhD candidate you will have to supply a research proposal along with evidence of your academic transcript. This all has to be approved by a graduate committee, the head of program and most importantly the main supervisor (if he / she is not interested in your project then that is the end of matters).

After you are approved then you sort of putter along and are at the mercy of the supervisors(s). If you get a good one then maybe they will help you towards a reasonable completion time which is usually viewed as 3-4 years. If you get a bad or average supervisor you will have to make do all by yourself and that is when things can get very problematic. Students in this situation can be “on the books” for years, sometimes suspending their PhD, sometimes not, sometimes bouncing between the paid employment and university life in a haphazard way...such people can languish in the system for years and only complete their PhD after 7 or so years. After 7-8 years on the books they may be kicked from a program. But in my experience programs hate to dump students and will keep them around.

The main factor in determining completion time is finding. In Australia it used to be relatively easy to secure a scholarship (if you are half way competent) from the Australia gov to the tune of 25k per year for 3 years with a possible extension of+6 months. In the Humanities (my area) this was considered pretty inadequate to complete a PhD and most people only complete after 4 years. So normally there was 1 or more years where people would be without money. Not very good, but normal.

You will also be thinking - what about paid employment in the academy? What about work as a university lecturer? As I understand the US system, work as a paid member of staff is built into the program. It is considered work experience. In Australia / New Zealand such work experience is not by any means guaranteed and in fact very few people will see any work as a lecturer. They may get some work as a tutor (TA) if they are lucky. This is because in AUS / NZ the PhD is seen as a training degree for research. It is not intended to provide a complete experience in the academic sector. The PhD only shows that you are capable of independent research, nothing else.

I say this as a way to illustrate how unstructured the PhD programs tend to be in this part of the world. This is both a flaw and a virtue. Flaw because the whole affair can be blown out to absurd time scales and folk are provided with qualifications that don’t really help them in the academic job market. But it is a virtue in that you have considerable freedoms as far as research is concerned. (Provided you don’t have an overbearing supervisor of course).

In addition to the other posted answer here, I'd also be curious to hear how this came to vary by discipline. I have a German doctorate in biology, which took me about four and a half years to complete (despite the advertised graduation time of three years, almost nobody in my program finished in fewer than four), but I've heard about people in humanities programs working on their doctorates for much longer, both in the US and in Europe. Surely there must be some statistics on how this has varied by time, by discipline, and by country.

While we are on the topic, can we also discuss the differences between undergrad and professional degrees?

I have effectively a law degree from Ireland. It is normally a three-year degree, after which one takes a year post-grad, take the mini-bar, then apprenticeship for a few years, then the bar. My wife got her degree in the US in...something not law related, then did a postgrad in law, and then took the bar. Which seems a hell of a lot less rigorous.

The other thing that stunned me is the emphasis on four-year degrees in the US. My dad has a BComm. In his day, and in my day, a Bachelor’s Degree in commerce is three years. Since we don’t spend any time on irrelevant minors, in practice it’s fine. However, and bless my fortune for having decided to do a different course afte all, which happened to take four years for the Bachelors, once I got to the US, the description “Bachelor’s” was irrelevant. “Four year degree” was what they wanted, the equivalency wasn’t relevant. Had I taken that BComm, I would never have been commissioned as an officer in the US Army. I get the feeling that a “three year degree” is pretty poorly rated around these parts.

There must be a reason for these differences.

In the sciences, American students typically enter PhD programs after completing their bachelor's degree. European students typically complete a master's degree beforehand. Total time spent in graduate school tends to be roughly the same. The first year or two of an American PhD program includes coursework, which European students will complete during their master's.

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So I think the question you really want to ask is a bit different, how did it arise that obtaining a PhD in the US is a single program, while it's split in two in Europe.

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Terminal master's degrees exist in the US of course, but are not the norm.