In a significant development in the mystery that has gripped Australia, NSW Police Commissioner Andrew Scipione and Premier Mike Baird announced the $1 million reward on Monday, the second anniversary of the three-year-old's disappearance.

It is double the amount of the state's previously highest standing reward of $500,000, attached to the 1999 case of murdered teenager Michelle Bright. 

Unlike other rewards that are contingent on someone being convicted of a crime, the $1 million reward in William's case is for his location.

William disappeared from his grandmother's yard in the small town of Kendall on the Mid North Coast on September 12, 2014. 

For the past two years the Homicide Squad's Strike Force Rosann has been leading the investigation into his disappearance and have been given information about 600 persons of interest.

Fairfax Media reported on Sunday that of the 600 persons of interest in the sights of police, 200 have not been completely identified.

Those profiles may include only physical descriptions from suspicious sightings and information gathered by police. 

In a bid to rule in or out each name or description on the list, information relating to about 400 persons of interest have been sent out to police local area commands across the state.

Police have previously said the three-year-old could have been the target of a paedophile ring in the north coast region.

The case has been referred to the NSW coroner but police say investigations continue with the hope the young boy is still alive.

- Ava Benny-Morrison

To outline the case so far, we have put together this special package including background stories, interactive maps, videos and photos. Click each photo to take you to those stories.

- Port Macquarie News