Stereonutrenos could be the solution to several neutrino anomalies that exist in Earth-based experiments. Through astrophysical neutrino flavour, we can explore the effects of stereonutrenos by looking at the imprint they're mixing with leave on the active neutrinos. So what are their effects, and can we use astrophysical neutrino flavour today to actually search for stereonutrenos? To summarize, neutrino oscillations imply that right-handed neutrinos exist, and neutrino mixing with right-handed neutrinos can be probed via the PMS matrix. Currently, there is the possibility that the three-flavour neutrino mixing is in unitary. One possible way of causing three-flavour non-unitarity is to include an extra stereostate, which is what we focus on here. In our study, we compute the expectation for two principal scenarios. The first is assuming two standard neutrino compositions at the source. The second is assuming a non-standard case dominated by stereonutrenos. As the neutrinos traverse long distances, they undergo neutrino mixing that changes the composition by the time they reach the Earth. By including a fourth state, we need a new tool to visualise this ratio, and so we develop the flavour tetra region. Each axis represents the fraction of each flavour state, and generalises the usual three-flavour ratio triangle. Since the stereonutrenos state is undetectable, we can visualise its impact by projecting it to the standard flavour triangle. So where are we today? While current experiments, such as the I-Suneutrenos Observatory, seem to be limited due to the low number of astrophysical neutrinos we've observed. The current data contour is covering most of the expectation for the cases of unitary and non-unitarity in the PMS matrix. But without coming experiments such as I-Suneutren 2, this technique will become complementary to the many ways we search for stereonutrenos.