  The relatively red wavelength range (4800-9300{\AA}) of the VLT Multi Unit
Spectroscopic Explorer (MUSE) limits which metallicity diagnostics can be used;
in particular excluding those requiring the [O ii]{\lambda}{\lambda}3726,29
doublet. We assess various strong line diagnostics by comparing to sulphur
Te-based metallicity measurements for a sample of 671 HII regions from 36
nearby galaxies from the MUSE Atlas of Disks (MAD) survey. We find that the
O3N2 and N2 diagnostics return a narrower range of metallicities which lie up
to ~0.3 dex below Te-based measurements, with a clear dependence on both
metallicity and ionisation parameter. The N2S2H{\alpha} diagnostic shows a
near-linear relation with the Te-based metallicities, although with a
systematic downward offset of ~0.2 dex, but no clear dependence on ionisation
parameter. These results imply that the N2S2H{\alpha} diagnostic produces the
most reliable results when studying the distribution of metals within galaxies
with MUSE. On sub-HII region scales, the O3N2 and N2 diagnostics measure
metallicity decreasing towards the centres of HII regions, contrary to
expectations. The S-calibration and N2S2H{\alpha} diagnostics show no evidence
of this, and show a positive relationship between ionisation parameter and
metallicity at 12 + log(O/H)> 8.4, implying the relationship between ionisation
parameter and metallicity differs on local and global scales. We also present
HIIdentify, a python tool developed to identify HII regions within galaxies
from H{\alpha} emission maps. All segmentation maps and measured emission line
strengths for the 4408 HII regions identified within the MAD sample are
available to download.
