This is a Tiger Swimming Snake, a relatively common snake that used to be considered non-venomous but is now considered venomous, but is weakly poisonous and not very harmful to humans.

Also known as the tiger-spotted neck-trough snake, it is about 80cm in total length. The back is bright green or grass green, with thick black patches on the front sides of the torso, and bright red between the black patches on the neck and some distance behind it