The Army is using a "B-team" to defend New Zealand from terrorism because it cannot find enough soldiers with the necessary toughness for its elite SAS unit.

The Counter-Terrorist Tactical Assault Group has been ready to combat a domestic terror threat since the end of June.

The group is under Special Air Service command but its soldiers have not passed the punishing selection course and had the subsequent training to be battle-ready for international missions such as in Afghanistan.

They are trained only in fighting the kind of urban warfare that would be needed in a New Zealand terror incident, such as the storming of a stronghold where hostages were being held.

The SAS is responsible for incidents the police cannot handle, but as only one in 10 applicants passes its the selection course, it has not been able to recruit enough members to adequately cover its increasing international missions and fulfil its counter-terrorist role at home.

It also has to replace staff leaving for other countries' special forces or lucrative private contracts in places such as Iraq.

SAS commander Lieutenant Colonel Peter Kelly told the Weekend Herald that lowering the standards of the SAS was out of the question, so the Counter-Terrorist Tactical Assault Group (CTTAG) was created.

"We would never lower our standards to get more numbers in," he said.

"If the SAS unit was fully manned, we might not need CTTAG. But the way things are, it is a great concept."

The size of the SAS is a closely-guarded secret, but estimates range between 80 and 150 troops. They have most recently been in Afghanistan, East Timor, Papua New Guinea and the Solomon Islands.

The CTTAG has 18 staff. Although officially part of the SAS, they do not wear the prized Special Air Service badge with a winged dagger and the motto "Who Dares Wins".

They wear the unit's sand-coloured beret, but have the same "non-badged" status as SAS support staff, such as clerks and drivers.

To join the SAS, Army, Navy or Air Force personnel must undergo a nine-day selection course described to the Weekend Herald by a participant as "mental and physical torture".

Among other exercises, its notorious fifth day involves 20 hours of non-stop marching in either a swamp or sand dunes while carrying rifles, a 20-litre jerrycan and a backpack.

Even if they make it through, they still might not be chosen for the nine-month SAS training course.

The CTTAG selection course is four days, followed by a four-month course.

The Army would not reveal details, but it is understood that although it is tough, it does not have a similar emphasis on "breaking" an individual. It has much less of a physical component, focusing instead on exercises that test teamwork, shooting skills and overcoming phobias such as fear of heights.

Twenty-eight members passed the first CTTAG selection course in July last year. Eighteen graduated from the training course in December, and after extra training in marksmanship they were deemed ready six weeks ago.

A smaller intake has just started this year's training course.

A source close to the unit told the Weekend Herald that although the CTTAG was a "B-Team", its members "are well and truly trained to do the business". But any operations would still be run by SAS members, "because a badged SAS member is simply not going to let a CTTAG guy go ahead of him".

Lieutenant Colonel Kelly rejected the "B-team" tag for the CTTAG, saying, "I would challenge anyone to see the difference."

He referred to them as a "bridging" or "halfway house" measure, introducing new soldiers, sailors and airmen to the culture of the SAS.

He hoped this would increase their chances of passing the selection course, revealing that the SAS had a pass rate of 12 per cent - just over one in 10 - and had rarely been able to man the unit to its capacity. "I think some of the chaps who came through CTTAG last year will have a go at SAS selection this year," he said.

"They live, sleep, work and play sport alongside SAS. They live the culture and the fitness regimes. It may be a little easier for them to make the decision to go for the selection course, although it will still be very hard for them."

Lieutenant Colonel Kelly said the international credibility of the SAS was based on its selection and training being measured against that of SAS units in Australia and Britain and special forces from the United States.

The source close to the unit said there was initial animosity towards the CTTAG from SAS members because they "had come in the back door" without passing the selection course.

This had since mellowed because the SAS members realised the CTTAG was assisting in getting them on overseas missions. Lieutenant Colonel Kelly acknowledged there were professional tensions, but said CTTAG members were not "second-class citizens".