Well in the first instance it would have to be if there is no detection by avast on a known infected site.
As I said there is no specific detection for phishing sites, but many such phishing site could also be malicious so there would be a degree of cross over. Avast has its own known malicious sites list, used by the network shield, there are many ways that it collects this data. One of them the web shield detections and community IQ after a period of time consistent web shield detections would result in the site being added to the malicious sites list.