Footer Contains White Area At The Bottom

On some of my pages which do not contain enough content, or some of the 404 pages such as (https://www.40squadron.org.nz/gfrg), there is white area right below the footer. I approached #wordpress and #css, they suggested I would need a javascript to auto calculate the lenght of the content or so.

Could someone please suggest how to fix this, or what script I would need?

Thank You