Holes on underside of bathroom sink...

[snags]

Got a blocked bathroom sink. Before undoing the trap and cleaning it out I thought a good plunge might sort the problem. Blocked the sink overflow. Filled sink with a few inches of water. On first downward plunge, water started to come out of a hole on the underside of the sink (there's one on each side of the underside). What are these holes for and should water come out of them when plunging? As you can tell I aint no plumber ;-p

[AlBags]

I expect they are connected at top of overflow.
Remove the plastic S or P trap and clear out .. as I expect that is the problem.
Usually scum and hair.

[AlBags]

When you put back together, get the black seals right and not twisted. Use fairy if you need to.
Order: Big plastic collar nut onto pipe, then the thin washer .. usually red, then push on black seal (taper towards end of pipe)

[ummmm]

Can fairies plaster?

[AlBags]

Well I can .. don't know about the fairy bit tho : )