Labour is slamming a new bill preventing coroners from investigating military combat deaths.

The Coroner's Amendment Bill is being debated as New Zealand prepares a troop deployment in the war against Islamic State.

National says it's neccessary for national security, but MP Phil Goff says it's a solution to a problem that doesn't exist.

"Wherever there is something that pertains genuinely to national security, the coroner can place a suppression order."

"Families, where they've lost a family member, really value having independent scrutiny outside of the military."