More than half of the KiwiBuild homes built since the flagship policy started were underway before they were brought into the programme.
National housing spokeswoman Judith Collins said this showed how little KiwiBuild was actually doing to add to New Zealand's housing stock.
"KiwiBuild is just a welfare scheme for property developers," she said.
But Housing Minister Phil Twyford said it made sense that the KiwiBuild houses completed first were the few that were already under construction.
In a written question to Twyford, Collins asked how many of the so-far completed KiwiBuild homes were not under construction before they were contracted to KiwiBuild.
In response to the question, Twyford said 33 of the 74 KiwiBuild homes that were reported as being complete, as of March 11, were not under construction before they were associated with KiwiBuild.
In other words, 41 houses – or 55 per cent – were already under construction before they became part of the programme.
"So far, more than half of the KiwiBuild houses were started before the Minister approved underwriting contracts giving guaranteed prices to developers," Collins said.
"That is not adding to the housing stock to count houses already being built."
But Twyford said KiwiBuild assesses whether the contract increases the supply of affordable homes overall.
In response to Collins' written question, he said bringing the houses which are already under construction into the KiwiBuild programme was the last step in what is frequently a months-long process.
"I am not prepared to demand developers put construction on hold while the legal formalities of them joining the KiwiBuild programme are concluded."
He said that going forward, the Government won't stop contracting partly completed homes into KiwiBuild.
"KiwiBuild will not ask developers to stop building homes while they go through the negotiation process."
In January, Twyford and Prime Minister Jacinda Ardern scrapped the KiwiBuild targets and announced a "recalibration" of the policy.
The Government had previously said it would build 1000 KiwiBuild homes by July 2019, 5000 in 2020, 10,000 the year after, and 12,000 every year after that until 2028.
"[Those] targets have not been a useful way to demonstrate our delivery programme, that's why the minister is looking at that again," Ardern said.
Speaking to The Herald, Twyford said he would be unveiling the new-look KiwiBuild in a few weeks.
But he said the recalibration would not include yearly targets, as it had previously.
"We're committed to the 100,000 over 10 years – but we won't be setting a new range of interim targets.
"We're committed to reporting transparently, as we have all the way along, about how we're doing right across the Government's build programme."