A young brother and sister who have not been seen since late last year may have been whisked away to the UK by their mother.

Thomas Kurt Michael Speath, 4, and his five-year-old sister, Serena Lucia Speath, were last seen when their mother, Jane Iluci Adare, collected them from their father's home in Brisbane on December 5.

The Federal Circuit Court issued a recovery order last month authorising police to recover Thomas and Serena.

The court said the location of Ms Adare and the children was unknown but they were likely to be in the southeast Queensland area, particularly the northern suburbs of Brisbane or the Maleny area of the Sunshine Coast hinterland.

It said there was some concern, however, that as Ms Adare is a UK/Australian dual national, they might have travelled to the UK.

The children have not been present at school and the Queensland Department of Education advised the court that the children were not enrolled at any other school in the state.

The court said there was a chance Ms Adare might be driving a new vehicle and the family might be travelling with a small white Chihuahua Pomeranian-cross named Pom Pom.

Jane Iluci Adare is a UK/Australian dual national. ( Supplied: Federal Circuit Court )

It has now made an order allowing publication of their details in the hope of locating them.

Thomas has short blond hair, blue eyes and is of medium build and approximately 117cm tall.

Serena has short blonde hair, blues eyes and is approximately 115cm tall and of slight build.

She is missing her upper right front tooth and her upper left front tooth is chipped.

She carries an Epipen auto injector for her allergies.

Their mother has light brown hair with some grey but it may have since been dyed blonde.

She has hazel eyes and a fair complexion and is approximately 165cm and weighs about 52kg.

Anyone with information on the whereabouts of Thomas and Serena are urged to contact Crime Stoppers on 1800 333 000.

AAP/ABC