Reddit mentions of New Life Spectrum Hex Shield 1mm Sinking Pet Food, 125gm

I second this. Nonetheless, I have encountered stock with stringy, possible parasite-caused poo and this product (fed for about a week) did the trick:
https://www.amazon.com/New-Life-Spectrum-Shield-Sinking/dp/B00RXQ5I3S

Will try ordering that, do you know or could recommend any food? Only issue might be that customs may pull it up or finding one that will ship here, but fingers crossed. Will try get some tetra safe start at the same time, its a long shot that it will still be viable by the time it arrives, but could be worth the shot.

This morning he was a lot perkier though, he came up directly for food, begging for it, and again when I saw him a lil later so it is a positive sign I guess even if he is still not looking physically better.

Edit* they won't ship any medicated supplies to NZ, but I did find a shop that stocks safe start so am going to give that a go once this is all clear and safe- and will be able to maybe get this it wouldn't arrive til Monday- but am being positive for the moment

White, stringy poop is usually an indicator of some sort of intestinal disease. Here is a link that lists symptoms and the recommended treatment.

In your case, due to the bloating and white, stringy feces, my best guess is Hexamita (see bottom of the link). The fish needs to ingest the medication so it goes through the gut. Hex Shield pellets (the smaller, 1mm size) might be a good option. If the betta doesn't go for it, try crushing the pellet with its normal food and see if that works.