I'm pretty sure more than 20 came to Oz now - there may have only been 20 in early 1976 (6 in QLD, maybe 7 in NSW, and 7 in Vic), but I think there was a second batch brought in to NSW, Vic, SA, WA later in the year.
The first batch seems to have been all around the 510-203100 engine numbers, but there is also another group of IT400C's out there with engine numbers around 510-205000.
So I think there must have been one lot came in in Jan/Feb 76 (the 20 I remember hearing about), and another batch later in the year.