The four Labour MPs vying for the party's leadership have faced off against each other in a televised debate today.

Grant Robertson, David Parker, Andrew Little and Nanaia Mahuta spoke about the country's security situation, policy and the future of the party on TV3's The Nation today.

The debate, hosted by Lisa Owen, began with discussion over Prime Minister John Key's warning of the increasing threat level facing the country.

Mr Robertson said the case for New Zealand to enter into an international conflict had not been made at this stage.

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"We simply don't know what the plan is, what the exit strategy is, and for him to use the kind of fear tactics that he has used up to now is something that I think a lot of New Zealanders are worried about."

Mr Little also said Mr Key had not made the case for New Zealand to put combat troops into Iraq to deal with the Islamic State.

Ms Mahuta said Mr Key should give New Zealanders confidence that if there was a threat of such a significance that he believed that military action was required, then that information should be in the public domain.

Mr Parker said during New Zealand's time on the Security Council we should be encouraging all nations to block the supply of arms, of money, of people to ISIS.

"I can agree that those things are appropriate to do. What I'm not convinced of is that it's appropriate for New Zealand to lend their weight to armed efforts.

"There's lots of humanitarian work that needs to be done, millions of displaced people. There's plenty to do without getting involved with the arms stuff."

All candidates said the economic gap between rich and poor was one of the biggest issues facing the country.

Mr Parker said: "Gaps between the haves and the have-nots are growing. The home ownership rate's the lowest in 60 years. Middle income people pay higher rates of tax than the wealthy."

Ms Mahuta said there were also other consequences of that.

"Inequality and poverty and the effects on vulnerable families, but also indebtedness of a generation of young person who wants to be a part of a productive economy but just can't quite reap all the returns because they've got huge student loans.

"The Government is creating an education system where there are winners and losers and eroding confidence in our public education system."

Mr Little said there were far too many people now who were dependent on low-paid, short-term, part-time work or multiple jobs to make ends meet and to keep their family together.

Mr Robertson said it was impossible to raise a family while earning minimum wage and house ownership was "getting out of your reach every single day".

Mr Robertson and Ms Mahuta both said they anticipated working with the Green Party in the future.

"I want Labour to be the strongest possible partner in a future government. But every government under MMP has needed a coalition partner," Mr Robertson said.

But Mr Little said come the next election he would be competing with the Green Party for every electorate vote.

"That doesn't mean I'll denigrate them, because they're not bad people, and we will probably need them in coalition, but it's Labour first."