  We present a detailed multi-wavelength study of an unobscured, highly
super-Eddington Type-1 QSO RX J0439.6-5311. We combine the latest XMM-Newton
observation with all archival data from infrared to hard X-rays. The optical
spectrum is very similar to that of 1H 0707-495 in having extremely weak [O
{\sc iii}] and strong Fe {\sc ii} emission lines, although the black hole mass
is probably slightly higher at $5-10 \times10^{6}~\rm M_{\odot}$. The broadband
SED is uniquely well-defined due to the extremely low Galactic and intrinsic
absorption, so the bolometric luminosity is tightly constrained. The optical/UV
accretion disc continuum is seen down to 900 \AA, showing that there is a
standard thin disc structure down to $R \ge$ 190-380 $R_{\rm g}$ and
determining the mass accretion rate through the outer disc. This predicts a
much higher bolometric luminosity than observed, indicating that there must be
strong wind and/or advective energy losses from the inner disc, as expected for
a highly super-Eddington accretion flow. Significant outflows are detected in
both the NLR and BLR emission lines, confirming the presence of a wind. We
propose a global picture for the structure of a super-Eddington accretion flow
where the inner disc puffs up, shielding much of the potential NLR material,
and show how inclination angle with respect to this and the wind can explain
very different X-ray properties of RX J0439.6-5311 and 1H 0707-495. Therefore,
this source provides strong supporting evidence that `simple' and `complex'
super-Eddington NLS1s can be unified within the same accretion flow scenario
but with different inclination angles. We also propose that these extreme NLS1s
could be the low-redshift analogs of weak emission-line quasars (WLQs).
