This paper investigates the mechanisms by which a health-promotion intervention might influence the health-promoting behaviours of staff members working in early childhood centres. The intervention was an ecological health-promotion initiative that was implemented within four early childhood centres in South-East Queensland, Australia. In-depth, semi-structured interviews were conducted with 10 early childhood centre directors, early childhood centre staff, and health-promotion professionals involved in the same initiative. The case studies were based on informal observations and a documentary analysis of health-promotion practices and activities. A thematic analysis was conducted on the data to reveal the key characteristics of the approach. This study found that the health-promotion intervention in early childhood centres was reliant on the development of collaborative, "relationship-focused" partnerships between health and community agencies and early childhood centres. These relationships supported early childhood centre staff to promote healthy and developmental appropriate behaviours such as communication and social connectedness between children, parents and the broader community. Collaboration between health and community agencies and early childhood centres led to the shared planning of curriculum activities designed to create supportive structures within the early childhood centre and home environments to promote the development of health-related behaviours. These findings demonstrate the potential value of an ecological health-promotion approach to support the development of health-promoting behaviours in early childhood centres. Although the direct impact of health promotion is difficult to assess, this paper addresses an important gap in the literature about how health promotion might indirectly contribute to the health in children by supporting childcare staff in their endeavours to promote health.