An apparatus and method to provide monitoring of pressure outside the wellbore casing of a well in which a Wireless Sensor Unit is placed externally of a section of non-magnetic casing. An internal Sensor Energizer Unit is placed inside the wellbore casing. The Wireless Sensor Unit includes one or more sensors to measure the pressure and/or temperature of the surroundings. The Wireless Sensor Unit and the Sensor Energizer Unit communicate using electromagnetic modulation techniques, and the Wireless Sensor Unit may be powered by means of power harvesting from the Sensor Energizer Unit.