Newnet had a similar problem, highlighted here, in 2011.Also a similar issue in 2010.
You should have information, in router, of all IP leases made by router recently. Are there any you do not recognise.
As others have suggested, turn off router for a period of time and check usage.
More inclined to believe this is a Newnet problem.
To blame your equipment is an easy cop out for them. My mac request would be on its way.
Edited by flippery (Wed 15-May-13 16:57:57)