Families of soldiers killed in the line of duty fear the truth behind battlefield deaths could be covered up under controversial laws being considered.

The new law is being debated as Kiwi troops prepare to depart for Iraq to join the fight against Islamic State.

Under the Coroner's Amendment Bill, which had its first reading last week, coroners would no longer be able to automatically investigate combat deaths, supposedly because it could breach national security. Instead the military would investigate deaths in-house.

Former chief coroner, Neil MacLean, has called the move undemocratic, and the partner of a soldier who died in battle believes it could lead to information being withheld from grieving families.

READ MORE: PM confirms Iraq deployment

Former army medic Sarah Erb, whose partner, Corporal Luke Tamatea, was killed in Afghanistan in 2012 when his Humvee was destroyed by a roadside bomb, said the coroner's report into his death had helped her and his mother get answers and gain closure.

"Without that report we'd feel quite ripped off, I think. That's our loved one and we should know what has happened."

There was an inquiry by the military into Tamatea's death but she didn't receive any information back from it.

At present a coroner can open an inquiry into a military death and can issue a suppression order if there are details that may risk national security.

But Government briefing papers say the amendment was prompted by concerns about the "appropriateness" of coroners investigating the deaths of soldiers in Afghanistan. Eight New Zealand soldiers were killed on duty in that conflict. Now coroners will only be allowed to investigate a soldier's death if the Attorney-General allows for it by determining the public interest and impact on New Zealand's security.

Defence Minister Gerry Brownlee said the changes wouldn't include self-inflicted deaths or training accidents but refused to comment further as the bill was before a select committee.

MacLean said the proposed changes were out of step with other countries.

"It flies completely in the face of what the Australians and English have been doing for a long time, which is real co-operation with the coronial system," he said. "Next of kin sometimes aren't too happy with military investigations. For better or worse our society has said it's important to have some independent judicial officer with sufficient power say 'what is actually going on here? What are the facts? Are there any lessons to be learned?' The moment you start to erode that power and say that's only if it's not a combat death, that's representing a watering-down of the role of the coroner." MacLean said he sensed a move to clip back coroners' powers.

Labour's Phil Goff said he fundamentally disagreed with the Government's actions. Families often didn't have confidence in a military Court of Inquiry and valued having someone independent examine the evidence, criticise and make recommendations.

"It raises the suspicion that it's being done more to avoid the Defence Force and Government being criticised when there may be good grounds for doing so."

There were other options that did not remove coroners' independent scrutiny but still allowed protection in genuine security matters, he said.

The NZDF supported the amendment and said it strikes a balance "between independent investigation and ensuring judicial scrutiny did not encroach into "matters of the state".