Twyfords $5000 is probably about right. Insulation for floor and ceiling around $2500 and the rest for a heat pump.
Insulating floor and ceiling is a no brainer but doing the walls in most properties isn't practical. It would effectively mean re-lining every room and no landlord would do that unless it was part of a complete renovation.
Double glazing is a good idea as well but whether most landlords would think it financially viable is another matter. You might be talking $15k-$20k for an average size house and your retired couple with a single rental to top up the pension won't have money like that available.
IMHO an end to Mom and Pop retirement rentals will only be a good thing for the property market in general. They’ve only done it as that’s the best way to invest once your own house is paid up, but I sincerely think it’s propped up the bottom end of the market and strongly influenced the current ‘unaffordable’ 1st home situation.
BUT... has anyone here considered the impact this would have on the low west end renters though... perhaps we’ll end up subsidising this in our taxes when they end up in Housing NZ because the lowest rental prices get out of reach instead...