if this is a retaining wall, you need to fix the problem ont he other side of the wall first. if this is just a wall, above ground on both sides, you can mix up some concrete patch and fill the hole back in. for that size vertical hole, you may need to brace a board over the hole to hold in the patch until it dries.

You should also consider using hydraulic cement, which sets up quickly and is meant to be used in wet situations where regular patching materials might be compromised as they set.

Hydraulic cement expands slightly as it sets.
It is not a good idea for a large patch since it can lead to cracking of the other material.
The speed of set is to prevent it from washing away before it can set (it can even make a set under water).
If the wall is not holding anything I would fill it in with some bricks, then use mortar to finish.
If it is a retaining wall you can still brick it up, but you must provide a mechanical lock for the patch to prevent it from being pushed out.

Or you could just shove some expanded metal in, support or attach it somehow, and smear on some morter or patch-crete.