numerous websites can no longer be reached on PC (using modem cable), eg amazon, imdb netflix
other devices using wifi can access these sites
firewalls checked and do not appear to be cause
not a NBN issue but appeared after it was connected
Welcome to TPG Community!
Since the sites are accessible to other devices, then there's no issues with your NBN service.
Have you tried a different browser to access the said sites? (Ex. Chrome, firefox, safari, etc.)
Aside from turning off the firewalls, were you able to clear the cache/browsing history of your browser?
May we know the error message that you are receiving?