For a design that hit the market in 1955 the modifications to meet the mid 1970s design rules were minimal. The only structural changes were the side intrusion bars. Headlight bulbs would have been changed to dip left I expect. (In the 1960s the DS had better headlights than any other car on the Australian market, and by late 1960s low beam was self-levelling and high beam turned into corners.)
It would have been necessary to fit ADR compliant seat belts - the English or french ones would not have been approved. And probably the unique Australian child restraints.
The DS was, as far as I know, the first car sold in Australia (and probably the first in the world) with a steering column which collapsed when the driver hit it, and the first with a deliberately designed controlled collapse rat front end.
They would also have been the first Australian built car with dual circuit disc brakes. (Many may not be aware that the DS and its variants were assembled in Australia in the 1960s.)

