The CWU, which has 300,000 members, built the with a bespoke content management system and a new infrastructure, to help cope with the relatively high volume of users and content downloads.
Kevin Slocombe, head of communications at the CWU, said: "Unions have to become more inventive and progressive in order to protect their membership and our new site responds directly to that challenge.
"The site has been developed with growth potential in mind and the flexibility to develop more effective, targeted and frequent communication with our membership."
The new site is expected to go live in November.
It was developed by Rippleffect, which is also behind a portal for Unite, the union formed by the merger of Amicus and the Transport and General Workers Union. Rippleffect will eventually develop a single Unite site, combining the information on the existing websites.