From what I can tell he is using paulownia, which is native to China but grown on plantations in Australia.
Supposedly paulownia is similar to balsa in its strength to weight ratio, but superior in other areas such as consistency of color, grain, density, etc. (if that is what one is after as some people prefer the variation found in balsa). Paulownia is also cheaper than balsa, which can not be grown on plantations and must be hand harvested (which makes it expensive).