The number of temporary social workers being employed by the local authority has increased in recent years because of high turnover of permanent staff, as well as long-term sickness among employees, according to Ofsted.
The regulator made its analysis in a damning report into the children's services failings in July.
Now, the corporate director of the department has said that agency staff are "valued but temporary" and that the council eventually want to have permanent staff in every position in the service.
Beate Wagner said: "The main expenditures that are draining the service at the moment is additional agency staff, and the other is placements (paying foster parents employed by private companies).
"There are advantages to having agency staff. We don’t have to pay them for sick leave or during holiday, but if all our current staff were permanent then I think we could save about Â£2m.
"If we had a choice then all of our staff would be permanent. That would be our first choice."
However, with the numbers of children in care rising and the council struggling to hire enough social workers, its reliance on help from agencies is likely to continue for the foreseeable future.
Ms Wagner said that one agency worker was currently filling a post in middle management while recruitment for that position continues.
She added: "We have had an increase in children in care, partly because there are some children who maybe didn’t get a good service the first time round.
"While we do try to intervene early to try and keep them from going into care, if they need to be in care then we have to make sure they are.
"It's not an unusual development at this moment in time."