I struggle to think of a diplomacy type role for an actuary in DFAT, analysis of stats maybe....
It would seem that DFAT wants people who know at least one language, have experience working overseas, did well at uni, committed to continuing learning, etc.
A rough-ish guestimate at a perfect applicant:
Achieved at least a D average at uni, in a course that has some international bearing.
Knows at least one language, more would be good.
Pursuant to linguistic knowledge has spent time immersed in foriegn cultures whilst working for NGOs or in the private sector.
Is physically fit, engaging in a martial art and a team sport for recreation.
Skilled debater.
Well rounded world knowledge.
Excellent interpersonal skills.
Skilled analysis, employing logical+lateral+etc thinking.
The stand-outs being a language and experience in the international sector. Neither of which are particularly suprising.
To give a leg-up I would recommend courses with an international bent esp economics/commerce/arts/law, Learning a language (not arabic, though market will be flooded), part of degree undertaken overseas on exchange, ANIP experience in public service esp DFAT, private sector or NGO involvement.
A good private sector exchange being things like the MITSUI exchange to Japan they take 25 well rounded, smart etc people to Japan immerse them in the culture for a while and in doing so build good-will.
As fall-back second rungs, consider ASIO, ONA, PMs department, Treasury, AusAid and that sort of thing or private sector/NGO.