Ground Water in Sewage Basin hole
We can presume that the basin is watertight (both out and in). The only real concern, then, is whether the boyancy of the air in the empty tank will overpower the weight of your concrete (or the mechanical connection between the concrete and the basin). 

If those aren't a concern, all should be fine. I imagine that the basin has edges and depressions that will anchor it well in the concrete.