Q. How do I reference, in the Harvard style, an online journal article that does not have page numbers?

If an online journal article does not have page numbers you would omit this information from the reference, e.g.

Graca, S. (2017) Domestic violence policy and legislation in the UK: a discussion of immigrant women's vulnerabilities. European Journal of Current Legal Issues, 23(1). Available from http://webjcli.org/article/view/531 [accessed 10 August 2018].