A coroner in New Zealand has been criticised for making a public submission against a proposed same-sex marriage law, whilst being involved in a decision not to hold an inquest into the death of a gay soldier who committed suicide.

Coroner Gordon Mantega was responsible for a decision not to hold an inquest into the death of Corporal Douglas Hughes, who committed suicide in April 2012.

Hughes had confessed his feelings for a fellow male soldier hours before. He then spent the time between his confession and suicide in the company of his commanding officer, who at one point frisked him for weapons.

His family have suggested an inquest should be held to assess issues they feel were not sufficiently dealt with by a previous inquiry, including the army’s processes for dealing with soldiers who are gay, and soldiers who express suicidal thoughts.

However, Mr Mantega ruled that there would be no further inquest into the matter.

A backlash against the decision began on Sunday after it emerged that Mr Mantega made a public submission to MPs against New Zealand’s Marriage Amendment Bill, which passed a first reading in August, in which he refers to marriage equality as an “experiment” and speculates on possible harm to children.

Mr Matenga’s submission read: “There is no evidence to suggest that children will not be affected by [same-sex marriage]. Until there is clear evidence that children will not be affected, the experiment should not proceed.”

His submission was criticised by the Attorney-General Chris Finlayson, who said such statements were “unwise” for officials in Mr Mantega’s position.

“In general terms, it’s pretty unwise,” said Mr Finlayson. “I’d expect people to be more careful about what they say and when.”

Mr Mantega’s decision was then backed by Chief Coroner Judge Neil MacLean, who said there was no connection between the decision on Corporal Hughes’ death and the submission on the Marriage Amendment Bill.

Labour defence spokesman Phil Goff has said that an inquest Hughes’ death should be held despite Mr Matenga’s decision, saying the possibility of anti-gay bullying in the army was not being taken seriously enough.

“Notwithstanding the decision of Coroner Gordon Matenga not to hold an inquest, circumstances surrounding the death and the concerns of the family mean this decision should now be revisited,” he said.

“Doug Hughes’ family believe that he was mocked and bullied about his sexuality and that nothing was done to help him.

The Defence Force must consider further whether its policies for dealing with the sexual orientation of its soldiers are adequate in light of this tragedy.

“There are good reasons why fraternisation on deployment is inappropriate and is not tolerated regardless of soldiers’ sexual orientation. But there also needs to be effective policies to deal with the circumstances which led Corporal Hughes to take his own life.”

The Hughes family have not made a public statement, but are expected to do so in the following week.

Prime Minister John Key said: “All I can say is I am confident that the military has a robust process for dealing with gay and lesbian employees, that they have a clear code conduct, and it’s my expectations that they honour their legal requirements.”

In January, three young gay men made submissions to MPs saying introducing marriage equality to New Zealand would reduce suicide rates.