Wong said the New Zealand dollar and weakening Chinese yuan had been "joined at the hip" since the trade war erupted last year.
Last month's surprise 50 basis point rate cut from the Reserve Bank had also helped to put the skids under the NZ dollar.
The Reserve Bank is widely expected to cut again in November to 0.75 per cent and market expectations are that the rate will drop to just half a per cent next year.
Currency dealers said the kiwi had fallen effortlessly fallen through what was seen as key support level of US65c, and just as easily breezed through the next support of US64c.
Wong sees more downside risk to the currency through to the end of the year.
Ultimately he put it down to the China-US trade war and its negative implications for world economic growth.
"If Trump capitulated and said 'let's get talking again', the Kiwi would be up by US2c in a flash," Wong told the Herald.
While good news for exporters - who stand to receive more New Zealand dollars when they repatriate export receipts from aboard, the Kiwi's dive makes imports more expensive.
"It's not good for NZ Inc to see our purchasing power fall like that," Wong said.
The kiwi put in a particularly poor showing in August, when it did worse than the yuan.
Since February, the currency has dropped by more than US6c, or 8.7 per cent, agains the US dollar.
The BNZ slashed its NZ dollar forecasts when it became clear that any US-China trade deal was unlikely to be agreed over the foreseeable future, following Trump's planned raising of punitive Chinese import tariffs.
Since then, a further round of tit-for-tat tariffs has emerged, raising fears about world growth and still lower interest rates.
Wong said that based on the tight relationship between NZ dollar and the yuan, the kiwi could gravitate towards a fair-value range of 0.60-0.63.
Based that relationship, the BNZ has pitched its year-end target at US61.5c.