Police have serious concerns for siblings, aged 11 and 12, who have been missing since Thursday.

There are serious concerns for two children who were last seen in southern Sydney more than 24 hours ago.

Luke Smith, 11, and Hayleigh Smith, 12, were last seen at a home on Willarong Road in Caringbah South around 4:30 pm on Thursday afternoon.

They are believed to be travelling with their mother, Nicole Boyle, and a man, Neville Kerslake.

missing siblings
Luke Smith, 11, and Hayleigh Smith, 12, were last seen on Thursday afternoon. Image via Facebook/NSW Police.

"There are concerns for the children's' welfare," Inspector Winston Pisani told reporters in Sydney on Friday.

hayleigh and luke missing
Luke, 11, and Hayleigh, 12. Image via Facebook/NSW Police.

The group is believed to be in the southern part of NSW and police have urged the children's' mother and Kerslake to contact the nearest police station.

Luke and Hayleigh are both of Caucasian appearance and thin build. Luke is about 105 cm tall, while Hayleigh is 150 cm.

mum man missing
The siblings are believed to be travelling with their mum, Nicole Boyle, and a 50-year-old man. Image via Facebook/NSW Police.

Boyle, 36, is of a large build, with brown eyes and brown hair, while Kerslake, 50, is about 175 cm tall, of Caucasian appearance, with brown eyes and grey hair.

"Police have information they may be travelling in a blue 2005 Ford Falcon with NSW registration DNJ 48E," a police spokesperson said in a statement on Friday.

Police are asking the public to be on alert and report any sightings of the children, woman, man or vehicles. Ring Triple Zero (000) to provide any information about this incident.