Say what you will about the man but Douglas was a pretty good writer. Not an honest one, but a good one: his books are great reading.
IOTL of course Rogernomics, as with all monetarist policies, was a huge failure. Some of the policies were certainly needed, but most was not. Ironically his best idea, universal basic income, suffered the exact same fate as Nixon's proposal. However Roger Douglas was considering stepping down in 1981, if Rowling wins Lange and Douglas will be marginalized as much as possible which may mean Douglas is out. Likewise avoiding the last government of Muldoon should leave NZ's books in better order, weakening the case for insanity.
That said Rawling had pissed off too many people, and a narrow '81 victory would do little to shut them up. I can't see how Lange doesn't topple him, myself, but Douglas is potentially long gone. However we can go deeper. Douglas himself only went hard-right in the early 1980s, and the only reason he pushed it through was because 1983-4 was a time of crisis in NZ. In this alternate timeline they probably follow a more Australian Hawke style plan regardless of Douglas being there.
So. Rowling wins 1981, scrapping into government with a narrow majority. This means several years to develop plans, no Douglas, no adoption of Douglas's plans (1982 IOTL), etc... which means they have time and potential solutions to their problems. If Rowling holds onto power, will he go much beyond 1986 or so? Even so Lange will now have Hawke's example in Australia, and can ignore the radical untested Douglas ideas (if he remains around).
Say Labour holds on in 1983-4, re-elected with Rowling or Lange, follows Hawke-like approach instead of crazy approach, the Nationals are pulled left instead of being able to double down with Ruth, etc... Economic success will likely lead to Labour returning in 1990, probably still under Lange without OTL's disasters under him and Douglas.
1993-4 however is more than a decade in power, I imagine the Nationals will take it back but Ruth won't get to nuke the rest of the economy. Pretty good for NZ all told.