If the pond is designed with a water level in the pond higher than the ground water level then the weight of the water in the pond will be more than enough pressure to act against the pressure of the ground water. To be able to get the EPDM liner in the hole, it really needs to be dried out of any standing water, so that the liner will settle on the bottom without filling it with water. Trying to pull the folds out of the liner is difficult enough with an empty liner, and impossible once there is an inch or more of water in it. To dry the hole, having a pipe large enough for a sub pump to go inside it slipped down one side on the deepest end will allow for the pumping of the water out, and as the water is being added to the inside of pond, the pipe can be raised slightly to keep more water in the pond than under the pond. If a pump can't keep up with the inflow, you may need to fill the hole partially to get to a level that can be pumped dry and go up on the sides making the depth up by going above ground.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard