It would run from St Marys to Campbelltown via Badgerys Creek.
Source: James, B. 1990. "Growth Suburbs Rail Link". The Daily Mirror, January 2: 7.
The current plans however would not see it extended through to Campbelltown, but rather to Narellan which is currently not served by rail.
Source: Transport Sydney. 2017. "Untitled" (Map). Transport Sydney, https://transportsydney.wordpress.com/tag/south-west-rail-link/ (Accessed September 3, 2018).
As for a real connection to Campbelltown, while not explored, a Y link could be built at Glenfield to allow for direct services from Campbelltown.
The Labor Party however would look at a through link to Campbelltown (perhaps via Narellan?). This would also connect to the soon to be completed Sydney Metro Line from Rouse Hill to Chatswood.
Source: The Daily Telegraph. 2017. Untitled (map). The Daily Telegraph, https://www.dailytelegraph.com.au/news/nsw/opposition-leaders-pledge-to-build-rail-links-connecting-sydney-suburbs-with-badgerys-creek-airport/news-story/19ec0773988f7fc9e02eac0fa7b11bba (Accessed September 3, 2018).