Some people asked if we were issued the correct callsign. Why didn't you apply for VK9NDX? That would sound much better. Or made the authorites a mistake?

I also was confused when I heard that VK9NDX is not possible. Here is the truth about constructing an Australian callsign with a 3-letter suffix:

Different from the 2-letter suffix the first letter of the 3-letter suffix stands for the license class you apply for.

Advanced VK$aa, VK$Aaa, VK$Baa, VK$Caa, VK$Daa, VK$Eaa, VK$Faa, VK$Gaa, VK$Iaa, VK$Jaa, VK$Kaa, VK$Saa, VK$Taa, VK$Uaa, VK$Waa, VK$Xaa, VK$Yaa, VK$Zaa, VK$RAN, VK2RAS, VK$WIA-WIZ (WIA), VK$GGA-GGZ (Guides Aust), VK$SAA-SDZ (Scouts Assoc.), VK7OTC, VK$IYA - IYZ (International years)
Standard VK$Haa, VK$Laa, VK$Maa, VK$Naa, VK$Paa, VK$Vaa
Foundation VK$Faaa
Repeater VK$Raa
Beacon VK$RSa, VK$RTa

The second letter of the suffix stands for the Territory:

C Cocos Island
L Lord Howe Island
M Mellish Reef
N Norfolk Island
W Willis Island
X Christmas Island

More information on that topic on the ACMA Homepage.