The Reserve Bank of New Zealand has confirmed it intervened to push the New Zealand dollar lower in August.The confirmation in the bank's monthly disclosure of its foreign exchange reserves triggered another one US cent fall in the currency to US77.5 cents. It has fallen almost four cents since Reserve Bank Governor Graeme Wheeler warned last Thursday the currency was "unjustifiably, unsustainably and extremely high," which was seen as barely veiled threat of intervention.The Reserve Bank said it sold a net NZ$521 million worth of New Zealand dollars in the month of August. This was its second biggest intervention in any one month since it sold NZ$1.5 billion in July 2007.It was significantly larger than the "passive" intervention expected by many economists, who had previously pointed to the last bouts of passive intervention seen in April 2013 and December 2012, when NZ$256 million and NZ$199 million were sold respectively.The RBNZ did not make a statement with the disclosure on Monday, unlike at the beginning of the bank's previous episode of large intervention in June and July 2007.However, Wheeler's comments on Thursday served as a justification for the intervention. His 2200-plus word statement, titled "Why the New Zealand exchange rate is unjustified and unsustainable" surprised many in the markets with its blunt and detailed comments about the extent of the New Zealand dollar's over-valuation. Wheeler pointed to RBNZ measures of the real effective exchange rate being 25 per cent over its average since 1964 and it cited IMF and Peterson Institute measures of the currency being anything from five per cent to one per cent over-valued."Unjustified and unsustainable are important considerations in assessing whether exchange rate intervention is feasible," Wheeler said. He added another factor to consider was whether financial market conditions were conducive to intervention.Meanwhile, newly re-elected Prime Minister John Key, a former currency trader, gave the green light for the intervention with comments at a Wellington media conference on Monday afternoon, shortly before the release of the Reserve Bank data confirming the intervention.Key said he supported the RBNZ's view that the currency was over-valued and that intervention would therefore be a logical next step. This contrasted with his comments earlier this year and last year that were sceptical about the effectiveness of intervention."I happen to actually support the view that the Governor has that the exchange rate is over-valued, so if they have intervened, that would be a matter for them, but it would seem fairly logical," Key told reporters.Asked what was a fair value for the New Zealand dollar, he said: "In the end the Goldilocks rate -- not too high, not too low, just about right -- I don't know, US65 cents maybe -- lower than it is today."ASB chief economist Nick Tuffley said the bank's actions reinforced the current governor's willingness to try something most did not expect and catch the market off guard. The central bank may intervene again to push the currency lower, he said."The RBNZ has essentially put its money where its mouth