The children had family contacts in the greater Auckland area and police believed this was where they were possibly headed.
A woman was seen leaving the property where the children were last seen in her own vehicle.
Police last night launched a major search and rescue operation on land and air as they look for 15-year-old Te Morehu McLean and his 7-year-old sister Anahera.
The young pair left their Potter Rd home in Tuakau around 10pm carrying a large suitcase, the little girl dressed in a bedtime onesie.
Now police are broadening the search area and even looking at the possibility the siblings were taken out of the area in a pre-arranged rendez-vous.
"While the pair left on foot we are looking into the possibility they may have been picked up in a vehicle by someone known to them," said Inspector Kay Lane.
"We believe they may be heading north towards Auckland where they have extended family and would like to hear from anyone who has information about their whereabouts that may help us find them."
Police were continuing to search the wider Tuakau area this morning and speaking with neighbouring property owners to check their sheds and outbuildings.
Police say they are concerned for the missing children's safety.
This morning Potter Rd resident Bert Schreurs told the Herald he heard a noise near his house between 10 and 11pm and got out of bed to investigate.
"I heard a door closing. I went outside and looked right around the house but there was nobody there."
Schreurs had also noticed fresh tyre marks on a gravel drive at his nearby unoccupied rental property.
He said the metal had been disturbed where a car appeared to have turned around.
However, he was not sure if it was from a search party or connected to the sibling's disappearance.
Nothing had been disturbed or taken from inside the house.
Schreurs said only a few houses were dotted along the dead-end rural road and it was generally very quiet.