It appears (but is not certain) that there were waves of immigrants to NZ, but this is not clear. Where are you getting a minimum founder population of 150 from. I have not yet run across any numbers for that.
This narrative in a popular reference work explains more, but I have no way to confirm its accuracy.
But if correct it seems that settlement was not a one time event, but rather there was a great deal of coming and going.
There is a list of references at the end that I am not interested enough to go through, but through which you may find your answer.
This article based on haplotype analysis suggests that the founder female population was 70 and it constitutes the overwhelming bulk of the native population. Note that since Euro arrival, the local Maori population is 40% white on average and its said there are no pure-bloods left.
There may have been some tiny gene flow on occasion. BTW, by the time the Euros arrived, Maori had long stopped using or apparently even remembering double hulled canoes and the single hulled dugouts were not seaworthy for the long traditional voyages. In the Pacific we really are talking about needles in Haystacks. Google Earth Samoa some time, its mind blowing! And its a 4 hour flight from NZ. Thats not far by Pacific standards.