Green’s Dictionary of Slang

Porirua briefcase n.

also Porirua sound system
[negative stereotyping of population of Porirua, a satellite town of Wellington, NZ]

(N.Z.) a large portable stereophonic tape deck/radio.

[NZ]McGill Reed Dict. of N.Z. Sl. 162: Porirua briefcase Portable sound system. 1980s.