Rietberg (17), from Groningen, scored 3311 points to beat Leesa Bathgate (Taieri) by 202 points.
Rietberg, who is spending a year at Logan Park High School, has competed in athletics for 11 years and is coached by one of The Netherlands' top throwing coaches.
Her best event is the shot put and this will be her best chance of winning a medal at the New Zealand secondary schools championships in Timaru next month.
She is also contesting the high jump and the javelin.
It was the most exciting contest of the multi-events championships which saw the lead changing hands throughout.
Rietberg had a personal best performance with her shot put of 10.47m that gained her 560 points.
Her other personal best performance was the 800m in 2min 46.73sec.
Her best points score of 610 came with a high jump of 1.49m.
Bathgate (18) led by a narrow margin of 18 points after four events at the end of the first day and stretched the margin to 63 points after the long jump that was the first event on day two.
It was only in the last two events that Rietberg gained the decisive edge with 439 points in the javelin and 498 in the 800m.
Bathgate's best score was 611 points in the 100m, 512 in the high jump and 522 in the 200m.
She also had a personal best shot put of 7.25m.
Danielle Lippitt (Hill City), a student at the Sports Institute of Otago, won the senior women's heptathlon title with 2242 points.
This included personal best performances in the 100m hurdles (22.18sec), high jump (1.22m), javelin (14.68m) and 800m (2min 38.18sec).
Dunedin school teacher Jarrod Adams (St Pauls) won his third Otago senior men's decathlon event, but he is not able to claim the title because he is now registered with the Southland centre.
His best events were the 100m in 11.88sec (675 points) and 400m in 53.24sec (671).
His total points were 4223.
John Gilmour (Hill City), who won the men's aged 16 and under octathlon last season, won the junior men's decathlon with 4949 points.
This included personal best performances in the shot put (9.83m), high jump (1.65m), 400m (55.62sec), discus (27.96m), pole vault (2.30m) and 1500m (5min 42.92sec).
The masters women's weight pentathlon was won by Winifred Harding (Leith) with 2688 points from Fiona Harvey (Taieri) 2547 and Claire Giles (Caversham) 2060.
Harding's best discipline was the shot put when she scored 713 points with her put of 9.22m.
Keith Hutton (Ariki) won the men's weight pentathlon with 3184 points from Scott McQueen (1731) and Nigel Alefosio (1365).
Hutton's best performance came in the weight throw when he scored 711 points from a throw of 12.80m.
In other events:Glen Ferguson (Leith) broke the 20-year-old masters men's aged 45 to 49 10,000m record that Allan Davis (Ariki) set in 1989 by 19 seconds with his time of 33min 29.77sec.
Sue Cuthbert (Leith) broke her own 3000m record in the women's aged 40 to 44 grade with a time of 10min 55.06sec.
Rosie Craven (Hill City) broke her own Otago girls aged 13 pole vault record with a clearance of 2.15m.
