Police said the woman in her 40s, two girls and a boy were all believed to have been shot.
All three of the children were aged under 10.
Police said they were searching for a man, aged in his 40s, who is believed to live at the property.
Officers have set up a crime scene at the property, on Lockhart Boree Creek Road, which they said would be examined by specialist police overnight.
They were called to the scene about 3:00pm and were continuing to search the property tonight.
A post mortem would be conducted to confirm the causes of death, police said.