This is a Koa tree, can't tell if its a Hawaiian Koa or the foreign Formosa Koa, need to take a closer look at the leaves and blossoms. Hawaiian Koa has very pronounced sickel shaped leaves and paler blossoms than the Formosan blossoms which are bright yellow.

Actually this is not a koa tree. Although it is a member of the same genus (Acacia) and exhibits the phyllodenous leaf form. This is actually Acacia confusa, invasive to hawaii and native to Southeast Asia.

I am a masters student who studies koa ecology at UH Manoa.

Here is a link that might be useful: Source