  We present high resolution [NII] 205 micrometer ^3P_1-^3P_0 spectra obtained
with Herschel-HIFI towards a small sample of far-infrared bright star forming
regions in the Galactic plane: W31C (G10.6-0.4), W49N (G43.2-0.1), W51
(G49.5-0.4), and G34.3+0.1. All sources display an emission line profile
associated directly with the HII regions themselves. For the first time we also
detect absorption of the [NII] 205 micrometer line by extended low-density
foreground material towards W31C and W49N over a wide range of velocities. We
attribute this absorption to the warm ionised medium (WIM) and find
N(N^+)\approx 1.5x10^17 cm^-2 towards both sources. This is in agreement with
recent Herschel-HIFI observations of [CII] 158 micrometer, also observed in
absorption in the same sight-lines, if \approx7-10 % of all C^+ ions exist in
the WIM on average. Using an abundance ratio of [N]/[H] = 6.76x10^-5 in the gas
phase we find that the mean electron and proton volume densities are ~0.1-0.3
cm^-3 assuming a WIM volume filling fraction of 0.1-0.4 with a corresponding
line-of-sight filling fraction of 0.46-0.74. A low density and a high WIM
filling fraction are also supported by RADEX modelling of the [NII] 205
micrometer absorption and emission together with visible emission lines
attributed mainly to the WIM. The detection of the 205 micrometer line in
absorption emphasises the importance of a high spectral resolution, and also
offers a new tool for investigation of the WIM.
