Authorship and reputation for British/Australian academic systems I have recently been offered a position for a "postdoc" (fixed term level A position) in Australia. 

In the British/Australian (and European) academic systems, career progression is very different from the United States, as outlined in this answer. 

In my field (a subset of engineering) we have an authorship order for publications based on contribution level, with the last author being the PI/Advisor of the lab (receiving a lot of credit in the process). 

In the British/Australian systems, there is an expectation to informally supervise a few students for post docs. I have been informed that I will be expected to help supervise or actually supervise students and perhaps obtain my own funding as well. In the US most post docs in my field exclusively focus on research, and nearly always get first author.

Additionally, if I'm moved to a B and/or C level academic (sort of like assistant and associate professor respectively here in the US) this supervision and funding responsibility will increase. This part is similar to the United States for a tenure track associate professor.

The difference between the US and the UK/AUS systems is that at the B academic level, I still would not be a professor, and I would have a PI above me. This brings up the authorship question...

Given the different structure between the two systems, does the UK/AUS system dilute the competitiveness and career advancement of junior academics in the UK/AUS system, especially if they intend to migrate back to the US academic system?

The reason that I ask is because in the US, once you are an assistant professor, you will get last author on any papers produced by your lab, but in the UK/AUS system, there may be a professor in charge of junior academics (assistant and associate professor equivalents) who would instead receive last author. This then dilutes the rating of the junior academics among their peers, because they become a middle author instead of the last author.

If this isn't the actual practice, please correct me.
Academics in Australia are usually on temporary contracts or on continuing contracts. Levels, however, don't directly relate to job permanency in Australia. For example, it's possible for an experienced postdoc (temporary, employed on PI's grant) to be paid at level B (starting rate is typically level A; step 6) and it's also possible for an academic to be appointed at level B for a continuing position (similar to tenure track). Although some level Bs will be on continuing positions, it's also possible for an academic at level C to not have a continuing appointment (e.g. a Future Fellow whose dept. has not committed to support him/her after the fellowship runs out). Yet they are still at level C because that is the appropriate level for their career stage (i.e. IF they had a permanent position, they would be appointed at level C). Most of these people, however, can leverage such a fellowship into a continuing position and start their own lab. This is similar to starting a tenure track position in the USA.

I think it's best to think of levels as representing career stage not the type of job (i.e. the level does not determine whether an academic is on a temporary or permanent contract, especially at levels A/B). You don't rise through the levels from a temporary contract to a permanent position - you must separately apply for a continuing position. This is analogous to how postdocs must apply for tenure track positions in the USA.

With regards to authorship, I think this is very group dependent. Typically postdocs run their own research and get first author while also supervising honours/PhD students and getting a second/middle author paper for that work. The way you describe this in your question (postdocs getting middle author papers but few/no first author papers) does not fit with my experience in Australia but it might occur in some groups. You could, however, find groups like this anywhere in the world. This is something you need to discuss with your potential advisor. 

Note: this post has been heavily edited from the initial version to address questions in the comments.