Police said the pair were riding toy plastic pushbikes in a back room of a house in St Huberts Island, south of Gosford, before they appear to have fallen into a swimming pool.

"It appears they were in a sunroom area and have gone along a pathway that leads to the pool," a police media spokeswoman said.

The pool, which is believed to be in-ground, has a fence around it but the gate may have been ajar, the spokeswoman said.

Family and neighbours tried to resuscitate the boys after they were discovered unconscious just before 7.15pm.

Police arrived at the Attunga Close home at 7.26pm and four ambulances arrived three minutes later to find the boys in cardiac arrest, a NSW Ambulance spokesman said.

"Police drove the ambulances so paramedics could remain in the back and resuscitation continued all the way to hospital," he said.

One brother died a short time later. The other boy was flown to the Prince of Wales Hospital in Sydney, where he remains in a critical condition today.

Neighbours, who rushed next door to try to help save the children, said the family had been in the house for a few years.

"I just think it's so tragic and they're a lovely family," one woman said.

"It's impacted quite heavily with all the people in the cul-de-sac," her husband said.

A resident in Attunga Close said the family was renting the two-storey brick house.

"It's a huge, huge pool," the woman said.

"I only met [the boys] a month ago when the mother brought them over for trick-or-treat.

"They were dear little boys, in their stroller, quite a handful I'd say but it's a terrible tragedy."

The incident comes less than a week after two brothers, aged four and 15 months, drowned in rough seas off Tathra, on the NSW south coast.

The toddlers' father also died after jumping off a wharf to rescue them.

A four-year-old Queensland girl also drowned on Saturday, when an inflatable 6000 megalitre water bladder burst at a weir on the Mackenzie River in Blackwater.

Police will prepare a report for the NSW Coroner.