Neighbour building a second house - my risks?

[HouseOwnerNZ]

Hi guys,

My neighbour is looking to build a second house on his land.
He needs to connect it to the public storm water pipes, so needs my consent to dig 2 manholes on my land (1 meter and 60 cm wide) and then connect them with a 22cm PVC pipe.

As the neighbour's new build implies a bit of excavation just 1 meter away from my house (manhole + pipe trench), my concern is a possible damage to my property (loss of lateral land support, which can lead to tilting/subsidence of the house and the surrounding soil).
The land is sloped (5-30 degrees in different places) and there is already some retaining construction where the digging is requested.
I do not want to be unreasonable, but looks like giving consent to my neighbour's project entails very expensive risks for me.

Here is the fragment of my neighbour's plan and a picture of the area (retaining construction where the digging is requested):
"https://drive.google.com/file/d/1rZIqZepI_mTKU0MbN9PtJcId22_RDyLa/view?usp=sharing"
"https://drive.google.com/file/d/1ef0f_ceZPMrY_lgkt4tyhJmng5tZyDM1/view?usp=sharing"

I should definitely seek professional advice (lawyer's and builder's).
Although I can refuse the consent, I cannot be unreasonable when doing so. I am ok with my neighbour developing his property as long as I don't risk expensive remediation.
I read on this forum that there is a legal provision to make neighbors give access to a public drain ("https://propertytalk.com/forum/forum/property-investment-forums/new-zealand/property-improvements-nz/41088-"). Not sure if this is correct though.

Would appreciate any experts' contacts or information/advice on the topic, please.
Thank you kindly.

[PC]

Will the new infrastructure prevent you extending or developing upon your own section?
Tell the neighbour your concerns. It's their problem to convince you.

[RollingCloud]

Look up Land Transfer Regs 2018 - Schedule 5 Rights and powers implied in easements. That's the not unreasonably withhold consent bit.
Dont pay a lawyer for something your neighbour requires. He will need and easement and you can be paid a consideration ($) for this. And the easement can have conditions in there to protect you. I suggest his lawyer drafts an an "Agreement to grant easement" which allows your lawyer to review at your neighbours cost along with all legal actions to effect the easement.

[HouseOwnerNZ]

Thank you PC and RollingCloud for your comments! Your input is really appreciated.
Answering PC's question, I am unlikely to build anything major in that area (like a sleepout or another dwelling) as it's already quite close to the edge of my section.
However, it will prevent me from improving the area, say, to be a quality recreation zone (bbq/patio). That's both construction-wise and aesthetically (with 2 manholes around).

[PC]

I'd say no then.
Maybe tell them that proposed solution limits your future options.
Funny how other options will appear when the cheapest, easiest (for them) is declined.

I've had a neighbour wanting to put a pipe through the middle of our back section.
The answer was hell no!
Instead they put the pipe in along the boundary to a different access point.