The Reserve Bank sold $NZ64 million in November, the largest amount since 2008.
Westpac said the central bank is acting on its mandate to manage its currency reserves by selling while the dollar is high, buying when it is low and making a return in the long run.
Westpac says although the amount was not large, the fact that the sell-off occurred is meaningful.
Senior economist Michael Gordon says the last time the Reserve Bank undertook this sort of currency trading was in 2007 and 2008, when it made hundreds of millions of dollars in unrealised profits.
He says the move is unlikely to influence the exchange rate, but any profit the Reserve Bank makes from the trading could improve the country's fiscal position because it would be returned to the Government as a dividend.
Mr Gordon says the Reserve Bank's latest move could be with a money-making objective in mind rather than any attempt to try and bring down the dollar.
He says it's in the spirit of what the Australian central bank has been doing for many years.
"They've taken the opportunity when their currency is high to sell a little bit and build up their reserves with the aim of selling it down and making a profit later."
Mr Gordon says that's in the spirit of going with the markets rather than trying to fight against them.