Shell.
I meant to reply earlier but got sidetracked into packing. I was thinking along the same lines as Beth i.e. you may want to discuss with your mother whether she could see a future living with you in NZ (not necessarily in the same house if you both value your independence
). It would mean a delay until you get your 3 years of PR under your belt but you say your mum still has 2 years left to work anyway.
If you yourself could see this as an option, discussing the possibility with her will certainly give her something to think about and also she will hopefully realise that you're not abandoning her but willing to include her in your future plans for NZ. I think perhaps some close relatives suffer from the feeling of being left behind and excluded when family members are setting off for a new life away from them and it could be that this hurt is the source of a lot of the upset and bitterness we often hear about. At least, if you give your mother the option of joining you, she will know the choice is hers and that she does have a choice. By the time you would be ready to sponsor her, she would have had some time on her own which will help her in her decision-making. Her life without you may take a different turn and colour her decision.
The regulations regarding parents visiting NZ on Visit Visas have recently changed although, I must say, I have still to fathom out how when they are offering stays of up to 18 months in a 3-year period but in 6 monthly lumps. It's a multi-entry visa but as far as I can see this facility is what we Brits can have now so maybe it refers to those from other countries who currently don't have this luxury. Perhaps it's no longer a case of having 6 months in NZ, then 6 months out before you can return and visitors could hop over to Oz and come back in on a new visa. I'll have to research this topic more closely when I have a moment.
I hope you and your mum can have a good discussion about both your feelings and come up with a resolution, even if it's only a temporary one. Nothing is set in stone so just go with the flow for now and let things sort themselves out in the fullness of time.