Context : Water vapour maser emission from evolved oxygen-rich stars remains poorly understood .
Additional observations , including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight .

Aims : We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H _ { 2 } O .

Methods : In its high-resolution mode ( HRS ) the Herschel Heterodyne Instrument for the Infrared ( HIFI ) offers a frequency resolution of 0.125 MHz , corresponding to a line-of-sight velocity of 0.06 km s ^ { -1 } , which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz .
Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut für Radioastronomie 100-meter telescope at Effelsberg , provided a ratio of 620.701 GHz to 22.235 GHz emission .

Results : We report the first astronomical detection to date of H _ { 2 } O maser emission at 620.701 GHz .
In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak .
At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component , jointly ejected from the star .
We observed the 620.701 GHz emission at two epochs 21 days apart , both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components .
Although we do not detect significant polarisation levels in the core of the line , they rise up to approximately 6 % in its wings .

Conclusions :