They gave up places in the New Zealand team for the Oceania Swimming Championships in Papua New Guinea, which finished on June 30, to focus on making the Youth Olympics qualifying times the week.
Teammate Hayley McIntosh has trained for the past year with the New Zealand high performance squad in Auckland and is working towards qualifying for the 2020 Tokyo Olympics.
McIntosh is ranked second in 800m and 1500m freestyle and third in 200m and 400m freestyle.
Smith is ranked first going into both 100m and 200m breaststroke, and second behind Commonwealth Games swimmer Bronagh Ryan in 50m breaststroke.
Simpson is ranked fourth for 200m backstroke, fifth in 100m backstroke and sixth in 50m backstroke.
Northland swimmers competing at the NZ Opens are: Bream Bay's Paul Linton (17); Northwave's Nellie Clark (14), Hayley McIntosh (19), Annabell Simpson (17), Ciara Smith (17) and Madeline Whittam (14).
The event will be livestreamed via the Swimming New Zealand Facebook page.