How long can the Kiwi keep flying?

The New Zealand dollar seems to be defying gravity of late, as the interest rate differential between the RBNZ's OCR and the Fed funds rate continues to move lower.  While the interest rate differential sometimes works with a lead and sometimes with a lag in setting direction for the Kiwi (aka the NZDUSD exchange rate), at present it is pointing to further downside.

This will only become more accentuated in the coming weeks as the Fed will almost certainly hike rates again in June.  Meanwhile the RBNZ expects itself to be on hold until late 2019.  So the question is, how long can the Kiwi keep flying?