If the proposed changes go ahead, business owners will simply save mail up to send in bulk on mail delivery days, O'Reilly said.
When a business did need to communicate urgently these days, it was highly unlikely to do so by post.
"I can't think why you'd use the post if the matter is time-sensitive," he said.
"And if it was a bill or statement, how many customers actually pay that as soon as they get it in the post anyway? One or two day's delay won't make a big difference."
O'Reilly said it was essential that a post system existed and if cutting days down meant NZ Post could stay afloat, that was a better option.
"It's better that it's fewer days in the week than nothing at all."
But Terry Hoskins, a trustee of the Independent Business Foundation (IBF), said he had "great concerns" about the proposals.
"Anything like this is going to slow the world of commerce down in New Zealand. This is a bad move."
Hoskins said he did not think NZ Post had thought the plan through.
"They've realised they need to save money and I think they're doing that for themselves rather than fulfilling their brief. They owe the country more than just being profitable themselves."
IBF will be making a submission on the changes, he added.
Any problems small and medium-sized businesses might face were "absolutely a consideration of NZ Post", said spokesman John Tulloch.
"We are aware of the issues and when we get to that point we'd be looking to engage with SMEs."
Tulloch said NZ Post would be working with SMEs to help them adapt, especially where a company had regular periodic statements to issue and pay.
"It will be a case of 'let's assist you to redesign your billing cycles," he said.