The order is effective immediately and the NGT has also asked the Delhi RTO to share data of the vehicles that'll be de-registered.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/ngt%E2%80%99s-diesel-vehicle-ban-what-it-means-you-and-auto-industry
[2] http://admin.indiaenvironmentportal.org.in/category/author/arpit-mahendra
[3] http://admin.indiaenvironmentportal.org.in/category/author/smriti-jain
[4] http://admin.indiaenvironmentportal.org.in/category/newspaper/financial-express-new-delhi
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/diesel
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicles
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/auto-industry
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/national-green-trbunal-ngt
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicle-technology
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution-control
[13] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution