Isla first made the top 100 girls' names in 2004 and 2020 is the first year it has topped the list.
Some parents may have been influenced by royals William and Kate.
Charlotte, the name of the Duke and Duchess of Cambridge's daughter, was the second most popular girls' name, while the name of their older son, George, was the sixth most popular boys' name.
Amelia is the third most popular name for girls.
Nikau was the most popular Māori name for boys, with 63 babies given the name, followed by Manaia and Ari.
The first name of director Taika Waititi made the top 10 list of Māori names for boys at number nine.
Mia was the most popular Māori name for baby girls last year, with 162 babies given the name, followed by Aria and Maia.
Nikau and Mia remain the most popular Māori boys' and girls' names in 2020, having topped the list for the past several years.
Taylor and Darcy are the most evenly split gender-neutral names for 2020, with a 51/49 per cent divide between boys and girls.
Despite their presence in the media, the names Jacinda, Judith and Ashley didn't make the top 10 list.
A total of 17,749 different first names were given to 58,676 babies born in 2020.
In 2019, there were 18,816 different first names given to 61,018 babies.
Top 10 - Boy
1. Oliver
2. Jack
3. Noah
4. Leo
5. Lucas
6. George
7. Charlie
8= William
8= Thomas
10. Hunter
Top 10 - Boy - Māori
1. Nikau
2. Manaia
3. Ari
4. Mikaere
5. Koa
6. Ihaia
7. Manaaki
8. Te Ariki
9. Taika
10= Mateo
10= Wiremu
Top 10 - Girl
1. Isla
2. Charlotte
3. Amelia
4. Olivia
5. Willow
6. Harper
7 = Ava
7= Lily
9. Sophie
10. Ella
Top 10 - Girl - Māori
1. Mia
2. Aria
3. Maia
4. Amaia
5. Kora
6. Kaia
7. Aroha
8. Anahera
9. Tiana
10. Kaea
