Three Perth children who died within 24 hours of falling ill had contracted a combination of a flu strain and a pneumonia-causing bacterial infection, the West Australian Health Department says.

The department's Communicable Disease Control director Dr Paul Van Buynder said all three children were under the age of five.

"It's apparent that all three of the children who had tragic consequences had very similar illnesses that started with a mild Influenza A infection and gone on from there to develop secondary bacterial complications,'' Dr Van Buynder said.

"Most of the children who have a secondary bacterial infection will develop a full-blown pneumonia and that will be part of the cause of the tragic consequences.''

Dr Van Buynder said an investigation by the health department had not discovered any links between the three dead children.

Worried parents have inundated emergency departments after the health authorities issued a warning on Friday urging parents to get children with fever and lethargy checked.

Double the number of patients normally seen were admitted to Princess Margaret Hospital's (PMH) emergency department, PMH Child and Adolescent Services executive director Dr Robyn Lawrence said.

Ten children an hour had been seen today, close to double the normal rate, with some parents waiting up to three hours to have their children checked by doctors.

About 21 children were admitted to the hospital yesterday which was about normal, Dr Lawrence said.

The health warning is still in place but Dr Van Buynder said parents whose children had had coughs and sniffles for some time should keep their children away from emergency hospital departments.

Dr Van Buynder said it was unusual for the flu strain and the bacteria, which is not uncommon, to kill children.

"Most years, despite the circulation of both the virus and the bacteria, we've seen no children who have this course of illness,'' Dr Van Buynder said.

Meanwhile, Queensland Health is urging all parents to vaccinate their young children against flu, especially if they also suffered from a medical condition such as heart disease or diabetes.

Health officials in Victoria said the number of flu cases were below average so far, while in NSW an initial increase in the number of flu cases had been reported going into the season.