According to orthodox rabbinical authorities, who wrote the Zohar Hadash?
In the same style as the Zohar; there are numerous Rabbis/Sages that give over teachings.
These Rabbis include Rabbi Shimon, Rabbi Yehudah, Rabbi Elazar, Rabbi Yitzhak, and others from the Zohar.
As far as to who actually recorded the teachings, in parshas Haazinu, Rabbi Shimon says: “I am arranging you as follows: Rabbi Abba will write, Rabbi Elazar, my son, will study orally, and the rest of the friends will converse in their hearts” (Zohar, Haazinu).
Therefore, it can be deduced that since the teachings in the Hadash all stem from the same era, that Rabbi Abba recorded those as well.