The headline read:

$15m scam revealed: US pair try to hoodwink Immigration NZ.

And it appears try they did, but as with many attempts to cheat the immigration, or any other, system it failed.
The criteria for Investor applications are freely available from Immigration NZ (INZ) or competent immigration advisers and lawyers, and the American couple should have been aware that:

  1. their funds had to stay in NZ, invested in approved investments
  2. there would be an audit of their investments at two years and again at three years before their residence applications would be completed.

That the Americans withdrew their applications before providing INZ with the evidence of their investments, actually suggests that, contrary to the inference made by the Herald and the political arguments by Labour, the system is working.

It is not the role of INZ to track investor activities on a day by day basis. INZ Instructions puts the onus on the applicants to show that their investments meet the criteria at all times and evidence is required to be provided when the next step in the immigration process occurs.

The responsibility for watching for unusual money movements lies with the financial institutions and the Police as happened in this case.
Whether or not the two Investor programmes are of long-term benefit to NZ is arguable but the government is focused on the short term foreign exchange funds they bring to NZ.