Transport only Z position

Hi,
I need a little help. This is my script:

this.transform.Translate(new Vector3(0, 0, 70));

Right now it wil transport the object with +70.
But how do I position the Z-position without transport the X&Y.

this.transform.position = Vector3(0, 0, 70);

This is what I want but right now the X and Y position are also changing.

Try this:

transform.position = new Vector3(transform.position.x, transform.position.y, 70);