James Macarthur died on 21st April 1867 and his daughter Elizabeth's husband Captain Arthur Onslow died on 31st January 1882. After the death of Sir William Macarthur on 29th December 1882 Mrs. Elizabeth Onslow inherited the Estate. She later suffered the loss of the wine industry through a phylloxera epidemic and weathered the financial crashes during 1885-1895 by selling of unpayable tenanted lands. In 1887 she took her children to England and studied developments in the dairy industry, returning to Camden in 1890. On 12th March 1892 she was granted by Royal Licence the right for her and her issue to use the surname Macarthur-Onslow.
Elizabeth applied the fruits of her own study of British dairying methods and the French métayager system of landlord tenant operation to put the property on a largely sharefarmed basis. Its new model dairies sold their output under a common brand through their own Camden Vale Co-operative Milk Company. This company in 1925 was merged into the Dairy Farmers’ Co-operative Milk Company which is still the Estate’s distributor. In 1899 Mrs. Macarthur-Onslow formed the business of the Estate into a limited liability company with her six children as the shareholders. She retained Camden Park House and 960 acres (388.5 hectares) as her own private property.
In 1865 a railway line was built through the eastern farms and a railway station set down at Menangle. The Estate’s headquarters were then moved from the Old Homestead to Menangle where a village school was opened in 1867.
In the 1890s the model dairies and creameries of Camden Park were served by 12 co-operative farms and 40 leased farms, while the products which included butter and cheese were taken by road to Sydney.