I was able to do a partial search of the NZ Rego database back in 2009 (running a "spider" that trolled the plates beginning "GA" through "GI", roughly April 1972 to April 1973), which found 257 Marinas, of which only 12 were still "live".
These are the oldest Marinas I found :-
(UK) GC6383 1275, First registered 11/03/1972, cancelled 29/10/1995, Engine No 12VEH48120, Chassis No MA45A510260IM
(UK) GC6332 1800, First registered 17/04/1972, cancelled 15/04/2002, Engine No 16045, Chassis No MH25980173M
(Oz) GF2759 1750, First Registered 06/07/1972, written off 29/10/1995, Engine No 110, Chassis No 595
And the oldest survivors
(UK) GE7520 1800, First Registered 26/07/1972, Engine No 18VEH45798, Chassis No MH4S9S126983M
(Oz) GC1682 1750, First Registered 25/07/1972, Engine No 20246, Chassis No 81373
Of the 247 that are still on the register, only 12 show with live plates
The third oldest is GG7286, Chris "Coupe+Rust" of Oamaru's Nissan powered Coupe (First Reg 26/09/1972, cancelled 11-10-1997). :P