Unless the changes went ahead, the letters business was likely to become unprofitable by 2018.
Mr Roche said despite the move to thrice-weekly deliveries, NZ Post would still be a six-day-a-week operation. "It's just that at the very front line the postie would deliver to household one on Monday and household two on Tuesday and then back again".
NZ Post also wants the deed changed to allow some of the 880 PostShops it is obliged to operate to be replaced with self-service kiosks.
But Mr Roche said it would be "imprudent" to guarantee there would not be further changes "because ultimately it depends on how people use the mail service".
"What we're saying is this model is sustainable for a number of years on current forecasts. If something happens outside our forecasts, an event where there's a rapid decline of mail, we would have to move quickly."
Mr Roche said the changes under consideration would allow NZ Post's letters business to run at a profit "for a number of years" by delivering savings of "several hundred million dollars" over a five-to-10-year period.
Raising the price of stamps was not an option as it was likely to increase the pace of decline in the business.
Proposals to replace some retail outlets with self-service kiosks would not affect Kiwibank branches, some of which shared premises with PostShops, Mr Roche said.
NZ Post employs 7000 to 8000 staff across its post and retail businesses including about 2200 posties, but Mr Roche refused to say exactly how many jobs would be lost until staff and unions were consulted but said it was likely to be "in excess of several hundred people".
NZ Post will also consult the public over the next six weeks before the Government decides whether the changes should go ahead.
"It will be a big change, a significant change and that's why we need to have a good conversation with the public", said Communications Minister Amy Adams.
Labour's SOE spokesman, Clayton Cosgrove, said it was "appropriate" that NZ Post look at its business model but it had to justify any changes and also consider the impact on businesses, on various groups including the elderly and on those in rural areas where there was no broadband.
How will the changes affect:
Posties
Posties are worried about their jobs - but not too much.
Cutting down deliveries would be a last resort because it would only accelerate the loss of business, said Postal Workers Union spokesman Graeme Clarke.
Parcel deliveries were growing alongside online retailing.
"I don't think it's going to diminish to nothing," Mr Clark said. "The change of mail volumes have been very quick in the last 15 years - who's to say in the next 15 years what retailing will look like?"
The elderly
Older people rely on post offices to pay bills and correspond with businesses and friends - and many would suffer at the hands of computerised self-service kiosks, says Grey Power.
National President Roy Reid said the plan needed more work.
"The elderly are not using computers and that sort of technology, and therefore we would expect [NZ Post] to provide a main counter where people can receive that service."
Self-service kiosks have been on trial in the Kapiti Coast, near Wellington.
Mr Reid said losing a couple of days a week of postal deliveries was unlikely to be a major disruption.
Farmers
Pockets of isolated back country are still connected to the rest of the world mainly by post. Broadband and cellphone coverage is poor.
"The last thing we want to see - or NZ Post would want to see - is a bunch of New Zealanders being isolated in some of those far-flung areas," said Federated Farmers national president Bruce Wills.
NZ Post had been in close contact and co-operative about its plans, and farmers were positive a solution could be reached.
Rural posties also delivered bread, milk, vet supplies and items to fix breakdowns. So there needed to be caution.
Public
In Auckland, most people said they would probably fail to notice if mail deliveries dropped to just three times a week.
"I just wouldn't even take notice of it," said Claire Allen, of Massey.
Ms Allen said she sent no personal letters, preferring email. And most of her bills arrived straight to her phone.
Aucklanders used post offices for 40,196 rates payments last year - just a fraction with more than 516,000 ratepayers in the region.
Paula Davis, of Titirangi, said she received the odd letter from her sister. But "it wouldn't matter if it arrived on Friday".
- Michael Dickison
Delivering change
* A state owned enterprise formed in 1987.
* Under the Postal Service Act 1998, which allowed competition in the letters market, a new "Deed of Understanding" required NZ Post to meet a series of social, stamp price, and service obligations.
* Under that deed NZ Post must provide five or six-day delivery to 99.88 per cent of delivery points or postal addresses.
* It also has to maintain at least 880 postal outlets and post centres.
* While letter volumes have fallen sharply in recent years, the number of delivery points has increased, squeezing profits.
* The letters business is now barely breaking even and is forecast to produce a $25 million loss by 2018.
