"This is the acid test for the Government. If National doesn't support this bill it will be clear they are readying more assets like KiwiRail for sale."
Labour Party leader David Shearer said Mr Cosgrove's bill was not too late.
"I think New Zealanders will be very supportive of the fact that we are putting in place provisions that will stop sale of other assets," he said.
"People will sleep better knowing the Government had to either go to a referendum or get 75 per cent of Parliament before they can sell off Kiwibank," he said.
United Future leader Peter Dunne dismissed the bill as a political stunt.
"I don't really think that it's feasible - I think that it's a matter the Government of the day should determine," he said.
He said United Future's pre-election policy was not in favour of a wholesale sale of state assets.
"We said if the Government put up the four energy companies and Air New Zealand with safeguards we would support that - it's only three per cent of the balance sheet.
"We ruled out the sale of Radio New Zealand, Kiwibank and ownership of water."