The gemara discusses whether the water that is poured on Succos (although the gemara implies that according to R. Akiva it is done even at other times of the year, but tosafos rejects this) is d'oraysa. The gemara concludes that it is in fact a halacha l'moshe mi'sinai. Tosafos (d.h. ishtamitsei) discusses whether there is any opinion who holds that it is merely d'rabonon. Tosafos says that according to the opinion that it is d'rabonon, it seems strange that chazal would institute a service done in the beis hamikdash with non-kodesh material. If the prohibition of bringing chulin into the azara is d'oraysa, they shouldn't have the ability to institute this, but even if it is only d'rabonon, Tosafos asks that it is awkward that they would institute bringing a service of pouring chulin water in the azara? Tosafos answers that since they were about to enter the rain season, they instituted this service as a means of mentioning the importance of rain from Hashem. It seems that this approach would only work to explain why they would do something so awkward as institute the bringing of chulin in the azara. But, according to the opinion that it is d'orasya, it would seem that chazal don't have the ability to institute nisuch hamayim, and that opinion would have to hold that it is d'oraysa.
Another point that comes out of this gemara is that although the Rambam writes that halacha l'moshe misinai cannot be debatable and there cannot be machlokes about it, here we have an example of halacha l'moshe mi'sinai on which there is a debate because there is an opinion who holds that it is not a halacha l'moshe mi'sinai and therefore only d'rabbonon.