Australian visa for a unique situation

I am travelling to Australia for about two months for work. I will not be working for an Australian company but rather for a European government entity, on a scientific endeavour in Australia, with the knowledge and cooperation of the Australian government. I will not be paid in AUD, and I will receive only my normal salary in my EUR account as well as a per-diem allowing us to recuperate the cost of a long hotel stay, which is to be done after we arrive back home.

I believe that this qualifies as a "business visit" insofar as:

Business visitor activities include: making general business or employment enquiries, investigating, negotiating, signing or reviewing a business contract, activities carried out as part of an official government-to-government visit, participating in conferences, trade fairs or seminars, as long as you are not being paid by the organisers for your participation.

I am a US national.

Is the correct visa for me the 601 ETA subclass?

Mostly I'm worried about the aspect of the per-diem and how this may affect the definition of it being a business visit.

I am also the first to land (small holiday first) so if any problems arise, they will arise with me. The rest of my colleagues are taking the EU-national version of this same visa.

What I can say to adequately explain my situation when I arrive at the border?

It would be stupid to say "I'm here for work!" even though I am there for work, I'm not there to work in Australia.