Allergen labelling

Can someone please advise if I can oversticker on a product to declare allergen status on the label that currently does not decalre any allergen or do I have to discard the whole label and re-print completely a new label with allergen declaration?

Not sure about Australia, but in the U.S. it is discouraged and many companies see it as a bad practice.  
 

Relabeling and Over-labeling

Over-labeling by placing a new label over an old label is discouraged by FDA but is acceptable as long as the new label and its use meet GMP requirements for attachment, legibility, reprocessing, and change control. (Over-labeling is also discouraged in some foreign countries.)

 

It would be best to replace the label due to the event that a second "sticker" could be pulled off, fall off, removed by rubbing, etc.  Then place a new updated label on the item... if that's possible.  The main issue is if someone wants to do your company harm they can pull the sticker off and then you would have an undeclared allergen issue...

Of course this is my experience with The United States and you local expectations, and the expectations of the locations you may export to, may be different.

I would agree with what was said above.