They were.

"Introduced diseases were the major reason for the Māori population decrease. In the 1890s the Māori population had fallen to about 40% of its pre-contact size. Decline accelerated after the Treaty of Waitangi was signed in 1840 and settlers began to arrive in greater numbers. This influx of people exposed Māori to new diseases, leading to severe epidemics."

http://www.teara.govt.nz/en/death-rates-and-life-expectancy/page-4