My daughters partner has taken my grandson interstate on the proviso that he would return within the week it now has bee 3 weeks and he keeps saying he will be back when he is ready .what legal right does my daaughter have to get her son home he is only 20 months old

She has right to file in court for emergency order.

Is the partner the legal father? Is he on the birth certificate? Are they married?

ANY question on law needs to include your general locale as laws vary by area.

Unless there has been a court order establishing custody, the legal father has equal rights. So the only recourse is to go to court and get an emergency order establishing primary custody with your daughter. Then she will have to take that order and go pick up the child. She may have to have the police accompany her to do so.