William Tyrrell suspects are being closely watched

Suspects in the abduction of William Tyrrell are being secretly watched away from the public glare of the country's most baffling missing persons case.

The man leading the hunt for little William, Detective Chief Inspector Gary Jubelin, said there had been a lot of activity play out in public - but there was much more happening behind the scenes.

"We're doing a lot of things that don't hit the public's attention and go just as hard on people. As I sit here, we're doing covert investigative strategies on a number of people," DEt Insp Jubelin told Tracy Grimshaw on A Current Affair.

There are almost 700 people in the sights of investigators. Some are of more interest than others, partly due to the way they had come to police attention.

"We base our investigation on facts, we don't speculate, [but] we come up with theories. At some stages some of our lines of inquiry have been played out very publicly."

Then three-year-old William disappeared from his grandmother's home in Kendall, on the NSW mid-north coast on September 12, 2014.

The case has dominated headlines ever since, from the confirmation a white goods repairman Bill Spedding was a "person of interest' to police fears a paedophile ring in the area could be to blame.

A $1 million reward for information was revealed yesterday - the largest amount ever offered in NSW.

"We're committed to finding out what happened to William. Sometimes there's a perception with the announcement of a reward police have run out of lines of inquiry."

Insp Jubelin said that was far from the truth.

"This is a very proactive operation. We are not going to sit on our hands and wait for someone to come forward, we are following numerous lines of inquiry," he said.

It weighed heavily on the minds of police throughout NSW that William hadn't been returned to his family.

"It doesn't sit well, it's unacceptable. Two years down the track [and] we don't know where he is."

Insp Jubelin told A Current Affair he believed William was still alive because there was no evidence to suggest he'd been killed.

It was a "hard" question to answer, particularly from William's parents.

"We have fears for him, for obvious reasons, but we haven't got evidence to suggest he's deceased."

Despite William being one of the most recognised faces in the country, Insp Jubelin said it was possible for William to be concealed, and called for people to be vigilant about the circumstances around a family's situation, for example if they have a young child with them that hadn't been seen before.

At a media conference with NSW Premier Mike Baird and NSW Police commissioner Andrew Scipione, Insp Jubelin said rewards were usually paid out on the arrest and conviction of the offender. On this case though, the recovery of William had been added as a condition to this reward.

"If you've got information regarding William's disappearance and you've got concerns that you could be potentially complicit in the offence, by concealing the offence, that offence will be negated the moment you come to police," he said.

The person's identity could be protected - and if they had fears for their safety, protection could also be offered.

"I'm confident it's only a matter of time before we find out what's happened to William and if we come to you the offer of the reward is off the table," he told reporters.

The large reward could now put pressure on the offender that they could be betrayed by someone they had confided in.