  From Swift monitoring of a sample of active galactic nuclei (AGN) we found a
transient X-ray obscuration event in Seyfert-1 galaxy NGC 3227, and thus
triggered our joint XMM-Newton, NuSTAR, and Hubble Space Telescope (HST)
observations to study this event. Here in the first paper of our series we
present the broadband continuum modelling of the spectral energy distribution
(SED) for NGC 3227, extending from near infrared (NIR) to hard X-rays. We use
our new spectra taken with XMM-Newton, NuSTAR, and HST/COS in 2019, together
with archival unobscured XMM-Newton, NuSTAR, and HST/STIS data, in order to
disentangle various spectral components of NGC 3227 and recover the underlying
continuum. We find the observed NIR-optical-UV continuum is explained well by
an accretion disk blackbody component (Tmax = 10 eV), which is internally
reddened by E(B-V) = 0.45 with a Small Magellanic Cloud (SMC) extinction law.
We derive the inner radius (12 Rg) and the accretion rate (0.1 solar mass per
year) of the disk by modelling the thermal disk emission. The internal
reddening in NGC 3227 is most likely associated with outflows from the dusty
AGN torus. In addition, an unreddened continuum component is also evident,
which likely arises from scattered radiation, associated with the extended
narrow-line region (NLR) of NGC 3227. The extreme ultraviolet (EUV) continuum,
and the 'soft X-ray excess', can be explained with a 'warm Comptonisation'
component. The hard X-rays are consistent with a power-law and a neutral
reflection component. The intrinsic bolometric luminosity of the AGN in NGC
3227 is about 2.2e+43 erg/s in 2019, corresponding to 3% Eddington luminosity.
Our continuum modelling of the new triggered data of NGC 3227 requires the
presence of a new obscuring gas with column density NH = 5e+22 cm^-2, partially
covering the X-ray source (Cf = 0.6).
