Woods from New Zealand are not especially well known for their strength. Not knowing what wood it is, is a very risky business. If your research indicates it could possibly be some sort of pine, it's not hard to identify the wood in the store. Keeping softwoods from heartwoods apart is not difficult. Google some images of radiata pine and compare that in the lumber yard. You could also just ask someone working in the store to identify it for you. If you still don't know the species, you can determine the specific gravity, which is a quick and pretty effective indicator of the strength of the wood. Measure the board and calculate the volume in cubic centimeters (cc). Weigh the entire board with a luggage scale (in grams). Divide weight in grams by volume in cc.
Still, since the chance is high the wood is radiata pine, I would never use it for bows. Avoid all pines in bows. An experienced bowyer might pull it off, but pines are generally at the bottom 10% of all potential bowwoods.