He said the three words "Lest We Forget" were known around the world to show remembrance of those who fought, and those who died fighting for freedom.
"Somewhere down the line in their [the thieves'] family history, there will have been a member who has gone to war.
"They are attacking their own family member and disrespecting all others who have lost their lives. It's heartbreaking."
With Anzac Day less than two months away, Mr Harold said it was crucial they had the sign returned before then.
"I can't think of what they will do with the sign.
"It seems pointless and no gain at all."
The sign was donated by D'Arcy and Jan Bailey. Last year it was taken down sanded, varnished and the lettering darkened before being put back in pride of place.
It would have taken at least two people to carry the sign and a vehicle to take it away.
Another wooden plaque bolted to a concrete wall less than 5m away was nearly taken by the thieves. Three of four bolts had been removed.
However, Mr Harold said the memorial rose garden plaque had now been removed and was in storage until the other sign had been returned.
Police had also been notified but Mr Harold appealed to anyone who may know where the sign was to return it the RSA.
"We want it back for Anzac Day, that's critical," he said.