  We present the HI eXtreme (HIX) galaxy survey targeting some of the most HI
rich galaxies in the southern hemisphere. The 13 HIX galaxies have been
selected to host the most massive HI discs at a given stellar luminosity. We
compare these galaxies to a control sample of average galaxies detected in the
HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001). As the control sample is
matched in stellar luminosity, we find that the stellar properties of HIX
galaxies are similar to the control sample. Furthermore, the specific star
formation rate and optical morphology do not differ between HIX and control
galaxies. We find, however, the HIX galaxies to be less efficient in forming
stars. For the most HI massive galaxy in our sample (ESO075-G006, $\rm log\
M_{HI}\ [M_{\odot}] = 10.8$) the kinematic properties are the reason for
inefficient star formation and HI excess. Examining the Australian Telescope
Compact Array (ATCA) HI imaging and Wide Field Spectrograph (WiFeS) optical
spectra of ESO075-G006 reveals an undisturbed galaxy without evidence for
recent major, violent accretion events. A tilted-ring fit to the HI disc
together with the gas-phase oxygen abundance distribution supports the scenario
that gas has been constantly accreted onto ESO07-G006 but the high specific
angular momentum makes ESO075-G006 very inefficient in forming stars. Thus a
massive HI disc has been built up.
