"We've had young kids who have accessed pornography accidentally on their phones and are quite upset about it. We've had kids [11 or 12 years old or younger] receiving sexualised messages or requests for nude pics."
For some the problems played out in violence or aggression at school, while others withdrew into themselves and struggled to learn or concentrate, she said.
But with no government funding available for in-school counselling until high school years, primary schools have to pay for it out of their already stretched teaching budgets, which only a few opt to do.
If children did not receive help early on, she was worried about where they could end up.
Ten New Zealand children between the ages of 10 and 14 committed suicide in 2014.
A 9-year-old girl killed herself last year in the bathroom of her Palmerston North school.
Violence in schools is also growing. In 2014, 71 students were excluded or expelled from schools for physically assaulting school staff, compared with 57 the previous year.
Nationally, primary school principals and the NZ Association of Counsellors are lobbying the Government to fund counsellors in primary schools.
But Maindonald said the need was even greater and more urgent in Christchurch, because community counselling services were so stretched already.
Even after an extra $20 million in government funding for Canterbury District Health Board mental health services, the amount available - $207 per person - was still well below the national average of $251 per person.
Home and Family Society Christchurch executive director Val Carter said the demand for its child counselling service had grown after the earthquakes, and was still rising.
There had been no waiting list for the service until this year, but now there were 26 children waiting to be seen, on top of the 101 children and 22 youth the organisation was currently working with, she said.
That was likely to be just the tip of the iceberg, Canterbury University associate professor Kathleen Liberty said.
She is conducting an extensive study following about 320 children through their first three years at school, and found at least one in five show symptoms of post-traumatic stress disorder.
But she said only about 8 per cent were getting any kind of counselling.
She believed having counsellors based within schools was vital because the problems were so common.
"Where there is an individual disaster, like a parent killed in a crash, the wider family and the school community can rally around to help. But when you have a community-wide disaster ... all the resources to help are stretched," she said.
Canterbury Primary Principals' Association president Jeanette Shearer said many teachers were struggling to cope.
"It's not just one or two children in a classroom anymore," she said.
"Some children just withdraw into themselves. They're not really engaging and they are not ready to learn. For other children there are behaviour issues and they become ... violent, and that's a real problem in the classroom."
She said Red Cross funding which paid for social workers in schools after the earthquakes had helped, but the programme's funding ran out early this year.
Issues at home, like family violence or a split between parents, could make a child more vulnerable to mental health problems, Maindonald said.
But many had supportive families and good parents, she said.
Often a child would talk to a counsellor about things they wouldn't tell their parents, she said.
"If they're aware the parent is stressed sometimes ... they will [shield things from them] because they're worried about them," she said.
She said it was a difficult time to be a child, and they needed somewhere to get support.
"I really feel for kids, because often families now are so busy trying to put food on the table just to survive. There are a lot of lonely kids out there, and they're more vulnerable," she said.
It was important to teach children ways to cope early, before they reached their teens and faced big decisions around things like drugs, alcohol, relationships and careers.
"If you teach these kids [how to] communicate about feelings and how to seek help, if those are embedded when they're young they're lifelong investments," she said.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
