Just like electrical appliances and wiring in your home or office, electrical equipment on an electricity network, such as transformers or overhead and underground powerlines has current flowing through it and produces EMF.
The level of magnetic fields from the electricity network depends on the amount of current/electrical load, the way the network is configured and the distance from the equipment. The level of magnetic fields is not directly related to the voltage. It is a common misconception that the higher the voltage, the higher the magnetic fields. For example, low voltage powerlines can produce higher magnetic fields than high voltage transmission lines.
It is also a common misconception that by installing powerlines underground the EMF is eliminated. The ground around an underground powerline provides shielding from electric fields, but it does not shield from magnetic fields.
Because underground powerlines are closer to ground level, they can have similar or even greater magnetic fields to the same powerlines installed overhead on poles.