Searching on metadata query syntax

[Emrys]

I want to search using a metadata field and cannot get a good result set.

I want to search for news articles with an exact match on the metadata

categoriesTopics = Rural and Regional Health Institute

This is the best I have been able to come up with:

https://search.csu.edu.au/s/search.json?collection=news&query=!NULL&profile=_default&sort=date&startRank=1&num_ranks=30&meta_categoriesTopics=[%24%2B%2B%20Rural%2Band%2BRegional%2BHealth%2BResearch%2BInstitute%20%24%2B%2B]

which returns
“fullyMatching”: 611,
“partiallyMatching”: 51

which is too many, and not accurate matches.