A mother is expected to be charged with the attempted murder of her 9-year-old son, as the search continues for her 5-year-old boy.

Deniliquin commander Superintendent Paul Condon told reporters today the case was "just terrible" and that "it will rock the community of Australia", news.com.au reported.

"It is a terrible story. It is a sad story. It's a tragic story. Not only for the community, the police but especially the family. It's just terrible," he said.

According to Fairfax in Australia, police will allege that the woman tried to drown her sons but was interrupted by a neighbour's dog.

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She allegedly told her family that they would never see the boys again and then travelled more than 70km from their home town of Deniliquin to the Murray River yesterday.

Nine News is reporting that the two children normally live at their grandmother's house but somehow ended up in a car with their mother.

They met with another man and went down to the riverbank.

Shortly afterwards, residents at a nearby property heard a scream. Their dog went running to the riverbank and they followed.

They allege their pit bull cross was taking the nine-year-old boy out of the water. But the boy's 5-year-old brother and mother had vanished.

There is speculation a pit bull was a hero dog that stopped the mother from allegedly drowning her son, but Supt Condon said it was unclear what role it played in last night's events.

NSW Police have applied for the extradition of the 27-year-old woman from a Victorian court.

Once granted the woman will be taken to Deniliquin where she's expected to be charged with attempted murder. She'll likely be refused bail overnight, to appear in Deniliquin Bail Court tomorrow.

Marine rescue NSW search the Murray River for a missing boy in Moama, NSW. Photo / News Corp
Marine rescue NSW search the Murray River for a missing boy in Moama, NSW. Photo / News Corp

"It is very unclear at this stage as to what happened at the riverbank. The dog has been involved. The dog has bitten the 27-year-old mother and the 9-year-old. The actual circumstances are unknown."

The owner of the dog, Buddy, did not want to be named but told The Herald Sun the dog was trying to save the boy from drowning.

"The dog is 4 years old and it's never attacked one person, has grown up with kids, and it's always been around kinds playing," she said.

She said her partner and his friend were on the property at the time and helped save the child when they found him in the river.

The pair "rushed the little boy to the hospital".

Meanwhile, searchers continue to comb the Murray River in Moama, on the border of NSW and Victoria, today looking for the missing 5-year-old.

When asked what the chances were the 5-year-old would be found alive, Supt Condon said: "There is every chance. We all live in hope".

However, he said searchers had not recovered any material from the riverbank, despite reports a small T-shirt was found.

Police were alerted by neighbours when the boy's brother, 9, was found with serious dog bite injuries about 7pm.

Supt Condon said neighbours found the boy and told police they had taken him to the hospital. Authorities realised the boy's mother and brother were missing and began a search.

Later the mother, 27, presented herself to police at Echuca in Victoria about 10pm and was taken to Echuca Hospital to get treatment for a dog bite. She has since been released.

"The 27-year-old female was interviewed by Deniliquin detectives this afternoon and they are now preparing documents for her extradition and charge of attempted murder," he said.

He said the woman was a Deniliquin resident who had two sons and confirmed she had been released from jail about a month ago for "several serious offences".

Lawyer Dale Brooks is representing the grandmother of the two boys and told the ABC she had approached several agencies with concerns about the mother but they "fobbed her off".
"A number of organisations ought to be held to account," Mr Brooks said.

Police spoke to the 9-year-old briefly last night.

"He required some surgery on the dog bites at this stage and of course, he is distressed.

He is a 9-year-old boy," Supt Condon said.

In an earlier statement, Supt Condon said: "Contrary to rumours, the dog was not set on the child. This has been an extremely traumatic scene for all the first responders involved and welfare support is being offered."

He said the dog belonged to a local family.

About 25 to 30 searchers are combing the river for the missing five-year-old from NSW and Victoria Police, SES and the rural bushfire service. Victorian Police divers have also been searching the river.

The ABC reported earlier the mother was expected to be charged with murder but the police spokesman said any further charges would depend on the results of the search.

"It is devastating," the spokesman said. "My police are devastated. The community is."