G-day Oliver,
A majority of the users on this forum are based in the north Americas region,
There is a section for Australia / New-Zealand on the front page.
In this section, and pretty much most other areas of the world we have the petrol versions of the disco.
I personally would love to have a Tdi in my Disco, I am British, living in Canada and respect the power and fuel economy of the TDi units.
I wish you good luck in finding a result to your question, It sounds like you are getting great economy to me, I have a v8 4.0 and it does approx 20ltrs to do 100kms. sh1t.
Current fuel costs in Canada in my region (ottawa) approx 86 - 89 cents per litre.
Perhaps I should move to NZ.

I wonder how far it is to drive to NZ, sure would take a hell of a lot of gas to get there
regards,
John.