Police said the children were dropped off to the Townsville police station on Tuesday afternoon but their father, Zane Smallwood, was nowhere to be seen.

The young children were believed to have been travelling with the 31-year-old man in an old Ford Falcon.

A police spokesman said officers still wanted to talk to Mr Smallwood.

Just before 2pm, they announced the children had been located and thanked people for their help finding them.

EARLIER

Police are investigating "instances and issues" five children missing in north Queensland may have been exposed to before they disappeared.

"Significant concerns" for the children, aged six, five, four, one and two months, sparked an Amber Alert when they disappeared on Monday, believed to be travelling with their father.

On Tuesday morning, Detective Senior Sergeant David Miles said police "know of" their father Zane Smallwood, 31, as a result of their current investigation.

"He will be hopefully assisting us with some inquiries that we have to make in relation to concerns in relation to the children but at this point in time our priority is the location of the children," he said in Townsville.

Senior Sergeant Miles wouldn't go into detail about why police held such serious concerns for the children's welfare.

"We have a current investigation running where concerns have been raised in relation to instances and issues that these children may have been exposed to," he said.

"As a result of that we believe that there are significant concerns for their safety and we need to locate the children and bring them into our custody for the purpose of conducting further inquiries and investigations."

Senior Sergeant Miles said the Amber Alert was activated as a result of information received overnight.

Police went to the family's Home Hill house, about 100 kilometres south of Townsville, about 8pm on Monday to check on the children but they weren't there.

They said the children had last been seen at the Third Avenue home on Monday morning and were thought to be travelling with their father, possibly in the Home Hill, nearby Ayr or Townsville areas.

Some members of the family had been helping police with the search, along with people in the community.

Senior Sergeant Miles said the mother was "not able" to help with inquiries.

He was investigating whether other members of the immediate family were helping Mr Smallwood, who police hadn't been able to contact.

"It's not beyond the realms of possibility that the children are with members of the family," he said.

Police said the car Mr Smallwood was believed to be driving was a 2002 Ford Falcon sedan with Queensland registration 947 WKD.

Call 131 564 to provide information about this incident or 000 if it's life threatening.