I checked with Air NZ and they say that if you change one leg of your ticket it costs US$75 (£50) but if you change umteen legs, as long as it falls in the overal mileage allowance, it doesn’t cost any more, if you do it at the same time.
So, I suggest, if you’re not sticking to the 18th June flight, forget about going to Bangkok on July 1st in order to hightail it to Singapore and then Bali on the 4th/5th; and, assuming you’re definitely decided Bali is ‘on’, change yout ticket (cutting out Kerikeri and Auckland) to go Auckland-Sin-Bali-Sin-London. Arrange the Auckland-Sin leg for whatever date Tan and Geoff arrive in Singapore. You’re already on the same flight as them (SQ146 at 1400 on 6th July) from Sin to Bali. Also, while you’re about it, try and get on the same flight as them out of Bali, too. Currently, you’re on Flight SQ141 leaving at 0915 on 25th July; they’re on Flight 147 leaving at 17.25 the same day. If you do that, you’ll have to change your Sin-London flight to link up with the Bali-Sin bit. All that’ll cost $75.
However, if you’re unsure about that routing, or about going to Bali at all, or what you want to do, just change the outward date from Kerikeri to somewhere in the distant future – say, November, or something – for the time being, until you’re more certain. That only costs a few NZ dollars, doesn’t it? There’s not much point expending $75 just to buy three weeks pondering time, when you may want to change it all again, later!
Having said all that, I am instructed by Mama to add that you can in fact go to Bali, and anywhere else, including the high mountainous rainforests of Borneo, any time …