Police were called to The Downs Road at 7.38am today to reports of a single vehicle collision.
The Royal Mail van had left the road and went down a steep embankment, just west of Newport Shute.
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The occupant, a man aged 40, from Southampton, died as a result.
His next of kin have been informed and they are being supported by specialist officers.
A spokeswoman for Hampshire Constabulary said: ‘Officers are investigating the exact circumstances of the collision and would like to hear from anyone who witnessed the collision, or saw the vehicle driving in the area prior to it.
‘In addition, if you have dash cam footage of the vehicle and/or the collision please call us.
‘Anyone with information is asked to contact police on 101, quoting incident 159 of today’s date.’