They were the only two Otago swimmers named in the 20-strong New Zealand team.
They will compete against Australia Green and Australia Gold at three North Island venues in July.
"This is great news for both the swimmers as they have worked very hard over the last year to achieve this selection," coach Andy Adair told the Otago Daily Times.
This is the first time since the early 1970s that the annual event has been hosted in New Zealand. The venues for the six competitions will be Mt Maunganui (July 5 and 6), Taupo (July 7 and 8) and Masterton (July 9 and 10).
Williams (17) and Gillespie (14) gained selection after winning gold medals at the New Zealand age group championships in Christchurch in March.
Williams won gold in the women's aged 17 and 18 800m in 8min 56.09sec, silver in the 200m backstroke and bronze medals in the 400m individual medley and 200m butterfly.
Gillespie won her gold medal in the women's aged 14 800m in 9min 06.80sec and silver medals in the 400m freestyle and 200m freestyle and a bronze in the 400m individual medley.
Williams also helped her selection chances when she finished fourth in the women's 800m in a personal best time of 8min 55.23sec at the New Zealand open championships in Auckland. It was worth 809 Fina points.
Williams finished the series on a high note by winning a silver medal in the 800m freestyle in Canberra in a personal best short-course time of 8min 41.45sec, 3sec faster than the time she swam in the first meeting of the series at Melbourne.
This swim was worth 838 Fina points.
Williams ended the series with six personal-best times from her seven swims and won a silver and two bronze medals.
NZ swimming - Transtasman team
• Men: Nathan Capp (Greerton), Matthew Hutchins (Wharenui), Shane Hornby (Thames), Jordan Quintal (Mt Eden), Alex Hancock (Howick-Pakuranga), Jeffrey Tuifanae (Howick-Pakuranga), Corey Main (Howick Pakuranga), Ewan Jackson (Howick-Pakuranga), Nick Gillum (Tawa), Julian Layton (Greendale).
•Women: Bianca Rowley (Coast), Alex Laidlaw (Aquagym), Sophia Batchelor (Aquagym), Elizabeth Brown (Ashburton), Josie Kydd (Ashburton), Natasha Lloyd (Kaiapoi), Renee Stothard (Howick Pakuranga), Phoebe Williams (Waves), Stefannie Gillespie (Waves), Shelby Wilson (Fairfield).
