Concerns for missing Sydney mum and kids

A mother and her four children have been missing from their home on Sydney's northern beaches for almost a month and police have serious concerns for her teenage boy.

It's believed Natasha Gow, 43, may be on Queensland's Sunshine Coast, travelling with her kids in a silver Toyota Yaris with NSW registration BJL 07P.

Ms Gow was last seen at her Narrabeen home on June 13 with Zacariah Fraser, 13, and his siblings - two girls aged 10, an 18-months-old toddler and a six-year-old boy.

"Police have serious concerns for Zacariah's welfare as he has a Type-1 diabetes and is not believed to have insulin," police said in a statement.

Zacariah's aunt is expected to speak to media on Thursday.