I don't think making 1981 worse helps - everyone's opinion was set anyway, and ratcheting up the violence would just lead to finger-pointing. Muldoon would order an enquiry that'd whitewash the police - an "orchestrated litany of lies, part 2." To get Labour winning 1981, I think you have to sabotage Muldoon's energy initiatives, or else have the Tour delayed until 1982.
To answer the question, you've got two options - either stop Douglas getting his paws on Finance or else stop him being the figure we know and love. The latter is actually really easy - Roger Douglas did not convert to neoliberalism until 1983 (he was a Minister under Kirk and Rowling, and was tribal Labour). The former means keeping Bill Rowling as Labour leader - also pretty do-able.
How about Colin Moyle never gets "stung" and remains an MP. There is no 1977 Mangere by-election, so David Lange doesn't enter Parliament (or if he does, it's much later). Rowling remains in place because no-one has the charisma to head a right-wing challenge. Rowling wins 1984, with Bob Tizard as Minister of Finance.