Each member of Parliament, each senator, has his or her own research staff. They also have research staff attached to their parties. Before the estimates hearings, those research staff do a lot of work preparing material for the senators. The committees also have their non-partisan permanent staff, and they're available to assist the senators in understanding the estimates and in formulating lines of inquiry.
Those staff are able to contact departments to clarify matters with departments beforehand and to signal to departments the matters they're going to be questioned on. Through that process, senators have a good deal of support and should be well prepared when they come to the estimates hearings.