  We report the detection of ammonia masers in the non-metastable (6, 3), (7,
5) and (6, 5) transitions, the latter is the first unambiguous maser detection
of that transition ever made. Our observations include the first VLBI detection
of ammonia maser emission, which allowed effective constrain of the (6, 5)
maser brightness temperature. The masers were detected towards G358.931-0.030,
a site of 6.7-GHz class~II methanol maser emission that was recently reported
to be undergoing a period of flaring activity. These ammonia masers appear to
be flaring contemporaneously with the class~II methanol masers during the
accretion burst event of G358.931-0.030. This newly detected site of ammonia
maser emission is only the twelfth such site discovered in the Milky Way. We
also report the results of an investigation into the maser pumping conditions,
for all three detected masing transitions, through radiative transfer
calculations constrained by our observational data. These calculations support
the hypothesis that the ammonia (6, 5) maser transition is excited through high
colour temperature infrared emission, with the (6, 5) and (7, 5) transition
line-ratio implying dust temperatures >400K. Additionally, we detect
significant linearly polarised emission from the ammonia (6, 3) maser line.
Alongside our observational and radiative transfer calculation results, we also
report newly derived rest frequencies for the ammonia (6, 3) and (6, 5)
transitions.
