  With steady observational advances, the formation of massive stars is being
understood in more detail. Numerical models are converging on a scenario where
accretion discs play a key role. Direct observational evidence of such discs at
a few au scales is scarce, due to the rarity of such objects and the
observational challenges, including the lack of adequate diagnostic lines in
the near-IR. We present the analysis of K-band spectro-interferometric
observations toward the Massive Young Stellar Object IRAS 13481-6124, which is
known to host an accreting dusty disc. Using GRAVITY on the VLTI, we trace the
crucial au-scales of the warm inner interface between the star and the
accretion dusty disc. We detect and spatially resolve the Na I doublet and He I
transitions towards an object of this class for the first time. The new
observations in combination with our geometric models allowed us to probe the
smallest au-scales of accretion/ejection around an MYSO. We find that Na I
originates in the disc at smaller radii than the dust disc and is more compact
than any of the other spatially resolved diagnostics (Br$\gamma$, He I, and
CO). Our findings suggest that Na I can be a new powerful diagnostic line in
tracing the warm star/disc accreting interface of forming (massive) stars,
while the similarities between He I and Br$\gamma$ point towards an
accretion/ejection origin of He I
