If it's monitoring the children's phone conversations, this may be more of a parenting issue rather than a legal one. The courts do no like to intervene on parenting decisions and will only do so in exceptional circumstances (e.g. where one parent is denying the other from seeing the children or trying to unilaterally take the children somewhere else).
If, however, what the parent is doing is not in the children's best interests, then I would suggest speaking with a family lawyer or your local community legal centre about this. Every family situation is different and so it is not possible to give you "our" opinion. As AllForHer has said, we can only tell you when court's may intervene, what factors courts will consider and, in exceptional circumstances (see examples above), when courts will likely intervene.