In an aircraft's pressurisation system is working normally the cabin pressure ( aka cabin altitude) varies dependant on altitude, it's not a fixed value in psi/bar/hectopascals throughout the flight.
a breach in the hull, it would mean the high pressure of the cabin would flow to the low pressure outside right
Correct, at high altitude there's a lot more more pressure inside than outside and in the event of a breach of the hull air will flow out.
if on the ground, and the aircraft is still pressurized for whatever reason. Wouldn't that mean the pressure outside the aircraft is MORE than the pressure inside the aircraft? So wouldn't it then flow in the reverse direction?
As you descend towards sea level the outside air pressure is increasing
but the pressure inside the aircraft is deliberately increased as well . FWIW if the pressurisation doesn't increase the internal pressure most aircraft have inward opening valves that open to allow air into the hull to prevent the hull from being crushed during the descent.
If the aircraft pressurisation system works properly when you land the pressure inside and outside the hull should be pretty much the same, but just in case many aircraft have a feature that fully opens up the cabin air exhaust, or outflow valves when the aircraft touches down, to equalise inside and outside pressure..