Just because it's illegal doesn't mean it isn't a classic, inventive, No.8-wire, bloke-in-a-shed response to New Zealand's isolation. In the 1970s the world was experimenting with psychoactive drugs, and that included New Zealand.
In 1974 there were just 24 criminal cases involving the most addictive of them, heroin. By 1979 heroin was much more available in New Zealand thanks to extensive drug trafficking by syndicates like the infamous Mr Asia drug ring. Then the New Zealand police cracked down. The Mr Asia ring collapsed and customs and the police strangled the flow of heroin into New Zealand.
As the 1980s dawned the heroin addicts of New Zealand were starved of what they needed - and their necessity gave birth to a weird antipodean bastard - homebake.