The eight hectare 'Campbell Rhododendron Gardens' at Blackheath in the upper Blue Mountains was established by the Blue Mountains Rhododendron Society and is maintained by volunteers. The site is much larger than this and has walking trails around the garden and bushland.
One of the things that struck me when visiting was the good effect of the exotic plants set within the Australian bushland.
April - May and September to November are likely to be the best times of the year to visit, but it is open all year.
The garden features a large pond and tranquil areas suitable for a picnic. Admission was just a 'gold coin donation'.