Updated 3.06pm: The largest ever investigation into our gambling sector has seen four men appear in court, charged in relation to an alleged 30 million dollar pokie fraud.

Prominent racing figures Mike O'Brien and his father Patrick, bankrupt hospitality consultant Paul Max, and a fourth man with interim name suppression have appeared in the Wellington District Court today.

The alleged offending was uncovered during an investigation called Operation Chestnut which involved the Department of Internal Affairs, SFO and the Organised and Financial Crime Agency of New Zealand.

Internal Affairs general manager for regulatory services Maarten Quivooy says Operation Chestnut has been significant.

"It's probably coming up to 18 months now, it's been a significant investigation but we are pleased to see the matter coming before the courts now."

Between them, the men face more than 20 charges of obtaining by deception.