The visual studio publish command rebuilds your solution and then copies the files in the solution directory to a new directory. I use the following target to do almost the same:
<target name="copyToPublish">
<delete dir="${dir.publish}" />
<mkdir dir="${dir.publish}" />
<mkdir dir="${dir.publish}\wwwroot"/>
<copy todir="${dir.publish}\wwwroot" includeemptydirs="false">
<fileset basedir="${website.dir}">
<exclude name="**/*.cs"/>
<exclude name="**/*.pdb"/>
<exclude name="**/*.csproj*"/>
<exclude name="**/obj/**"/>
<include name="**/*.*"/>
</fileset>
</copy>
<mkdir dir="${dir.publish}\database"/>
<copy todir="${dir.publish}\database" includeemptydirs="false">
<fileset basedir="${dir.databasescripts}">
<include name="**/*.sql" />
</fileset>
</copy>
<xmlpoke
file="${dir.publish}\wwwroot\Web.config"
xpath="/configuration/system.web/compilation/@debug"
value="false" />
<xmlpoke
file="${dir.publish}\wwwroot\Web.config"
xpath="/configuration/system.web/trace/@enabled"
value="false" />
<move file="${dir.publish}\wwwroot\Web.config" tofile="${dir.publish}\wwwroot\Release.config" overwrite="true" />
<delete file="${dir.publish}\wwwroot\Web.config" />
</target>
Before this target you have to run the normal build procedure of course.