How to change MySQL GROUP_CONCAT to PostgreSQL string_agg?

I'm migrating some MySQL code to Postgres and having a heck of a time converting the following line:

GROUP_CONCAT( DISTINCT id,':',foo,':',bar ORDER BY id ) as id, 

This results in a comma separated list of strings like:

id:foo:bar,id2:foo2:bar2

The 'DISTINCT' is there to avoid duplicates.

I've read the equivalent of GROUP_CONCAT in Postgres is string_agg, but I can't figure out how to make it work the same way.

Edit: I may have almost answered my own question. I came up with the solution of using CONCAT. The problem with this is that now sorting is done by the concatenated string rather than by the id:

string_agg(DISTINCT CONCAT( id::text, ':', foo, ':', bar, ':' ), ',') as id 

If I try to add 'ORDER BY id' I get an error.