I live near what used to be several of these - Kaiapoi Island,Coutts Island, McLeans Island, Templar Island, no doubt forgetting some.

Created by the Waimakariri River splitting into 3 branches as it neared the ocean.

They're no longer islands as the two side-branches of the river caused repeated severe floods in colonial days, so large civil engineering efforts went into restraining the river, ultimately leading to the south branch and north branch being blocked by large stopbanks and riprap cassions.

Both branches were then utilised to drain water from high fertility but swampy ground.

Because NZ's braided rivers lend themselves to wandering channels, we've still got some decent sized river islands - Rākaia Island, Rangitata Island.