Is 30 mins exercise a day really enough to lose weight?

I have just joined a 12 week challenge at my local gym, my goal is to shape up and hopefully lose around 10 kilos in this 12 weeks (from 67kgs to 57kgs..) The Personal trainer has told me I don't have to do anymore than 5 session a week with him for 30mins at a time.. This seems too easy? (the workouts are hard but only 30mins.. can I seriously expect to lose wieght doing this) he seems to think the 10 kilo's is achievable.. The Reason why I am skeptical is Because I already do 3 hours of kickboxing a week for the past 2 years and seem to have put on weight since doing this??(and it's not muscle unfortuantly) He checked my diet and said that it is fine and healthy.. just up the water intake.. and maybe make my dinners slightly smaller(only eat 3/4 of what I have now)... Will this work?