mvn release:perform does not inherit command line variable

When I deploy i must add -Duser.name=mickw to ensure scpexe logins in with the correct username.
eg mvn deploy -Duser.name=mick

But when I run "mvn release:perform" scpexe fails when it forks the sub process:
mvn deploy site-deploy --no-plugin-updates -P null,null -DperformRelease=true

and it's missing the -Duser.name=mickw variable.

I need some way of setting the user.name property to make it possible to run release:perform