  Centaurus A is the most nearby powerful AGN, widely studied at all
wavelengths. Molecular gas has been found in the halo at a distance of ~20 kpc
from the galaxy centre, associated with HI shells. The molecular gas lies
inside some IR and UV bright star-forming filaments that have recently been
observed in the direction of the radio jets. These archival data show that
there is dust and very weak star formation on scales of hundreds of parsecs.
  On top of analysing combined archival data, we have performed searches of
HCN(1-0) and HCO+(1-0) emission with ATCA at the interaction of the northern
filaments and the HI shell of Cen A. Measuring the dense gas is another
indicator of star formation efficiency inside the filaments. However, we only
derived upper limits of 1.6x10^3 K.km/s.pc^2 at 3 sigma in the synthesised beam
of 3.1".
  We also compared the CO masses with the SFR estimates in order to measure a
star formation efficiency. Using a standard conversion factor leads to long
depletion times (7 Gyr). We then corrected the mass estimates from metallicity
effect by using gas-to-dust mass ratio as a proxy. From MUSE data, we estimated
the metallicity spread (0.4-0.8 Zsun) in the filament, corresponding to
gas-to-dust ratios of ~200-400. The CO/H2 conversion ratio is corrected for low
metallicity by a factor between 1.4 and 3.2. Such a low-metallicity correction
leads to even more massive clouds with higher depletion times (16 Gyr). We
finally present ALMA observations that detect 3 unresolved CO(2-1) clumps of
size <37x21 pc and masses around 10^4 Msun. The velocity width of the CO
emission line is ~10 km/s, leading to a rather high virial parameter. This is a
hint of a turbulent gas probably powered by kinetic energy injection from the
AGN jet/wind and leading to molecular gas reservoir not forming star
efficiently.
