Elections can be a dirty business but it was stepping off the campaign trail that saw defence minister Mark Mitchell covered in mud today.

The Rodney electorate MP took a day away from chasing votes to take part in the "longest day" for trainees involved in the NZ Defence Force-led Limited Service Volunteer programme.

The scheme takes on 800 trainees aged 17-25 who are receiving a benefit and aims to teach life and motivation skills and get them prepared for training or work.

Trainees include those involved in crime, others who have served time in prison and some wrestling with drug addiction.

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But there are also those who have simply fallen into a rut and struggle to get up in the morning.

Mitchell joined the trainees for the "longest day" - a signal moment in the six-week course which has the trainees up before dawn and competing against each other through a series of gruelling challenges.

Mitchell joined the trainees in Auckland's Green Bay before first light with the "Jake the Snake" challenge at 5.30am.

The challenge takes place in the dark and has trainees having to hold and follow a rope along a small bush track with a series of obstacles and pitfalls.

It leads into other challenges during which groups compete against each other for the best time, finishing around sundown.

Typically, there is a mud run - and Mitchell was caked from toe to thigh with smears of mud the length of his body to the forehead.

Mitchell was initially invited in 2011 as some of the trainees were in his electorate. "I fell in love with the programme," said Mitchell during a break. "I try and come as often as I can."

While Mitchell turns up to meet trainees regularly, it was his fourth "longest day" with his involvement in the programme.

The trainees tended to turn up with low self-esteem and suffering a lack of direction, he said.

"They're put into an environment where they get some of the best leadership and mentors in the country."

The programme runs on a strict military discipline basis with trainees issued uniforms and expected to address NZDF personnel by rank.

Mitchell said trainees left "with pride they have in each other and respect".

Trainees are supported through the course - which runs 10 times a year in Auckland and Christchurch - by a social worker, clinical psychologist and a registered nurse.

The National Party has said it will run a year-long programme for young offenders aged 14-17 at Waiouru military base called the Junior Training Academy.

Mitchell said he expected the academy would "take all the good things from the LSV programme and use that in the junior academy".

Wing Commander Tua Atkinson said about 80 per cent of the 800 trainees annually completed the course and of those, about 80 per cent went into jobs or training.