Thanks for the review.
Collision detection was an issue we had throughout development. We wrote the collision detection ourselves in our game library rather than let Unity do it for us so we could port this to another game engine if we wanted.
All the math and unit tests shows that the hit detection is technically correct but we also felt something wasn't quite right.
We ran out of time to fully resolve it. I think the issue is the that code is right but the sprites aren't drawn quite in the centre of the sprite images. Also you often get hit multiple times by very close together zombies. We should redo the sprites to make them narrower with a narrower hit box and also give the player half a second of immunity after being hit.
Note that if you time your jump up right you can "jump through" zombies jumping down and vice versa - this is intentional.