They left a note, packed their bags and ran away together "to be away from everything".

At just 14, Lucas Bollard and Sophie McGregor have left their parents and police worried about where they've been for three days.

The Sydney teenagers, who have been in a relationship for three months, haven't been seen since Saturday, police said.

Lucas's parents called police after finding a note he'd left for them.

Botany Bay local area command duty officer Inspector Mal Smith said the young couple had taken enough money to "get by", but their disappearance was concerning because of their age.

"We believe they're two young persons who may be in love," Inspector Smith said.

"They only met three months ago apparently.

"It's apparent to us at this stage they may have gone away to a location to be away from everything."

Police believe they may have gone to the Blue Mountains or Lithgow and are asking anyone who sees them to call authorities.

Both teenagers were from stable backgrounds, Inspector Smith said.

"It's just out of character for both of them and we're just making all the inquiries we possibly can.

"They're young kids trying to get away and our concern is that we don't know where that 'away' is."

Inspector Smith said it's unclear exactly what the teenagers packed to take away with them.

Lucas, who is 183cm tall, was last seen wearing blue jeans, white sneakers and a grey Hurley hooded jumper.

Sophie, who is 152cm tall, was last seen wearing black jeans, black boots and a white t-shirt with a cartoon on the front.

She may have been carrying a blue cardigan and had a green computer bag with a tree motif on the front.

A NSW Police spokeswoman said officers would be looking at whether the pair had mobile phones with them or had updated their social networking sites.

- Sydney Morning Herald