How can I use toggle bolts for kitchen shelves on 30 mm plasterboard?
There is an anchor that has similar characteristics to a toggle bolt, known as a toggle anchor



These have a long straps holding the metal backing toggle. The straps are slid out of line to turn the toggle parrallel for insertion in the hole. Then the straps are aligned to turn the toggle perpendicular. The outer collar is slide along the tracks to the face of the wall and the excess straps are broken off. 

This has a significant advantage over spring toggles because the bolt can be removed without losing the toggle.

If the shelf is heavy, be sure to use numerous anchors to distribute the load. Plasterboard is not a very strong support.  And see Tester101's comment above.

You can see a general discussion of different types of anchors here.