The Serious Fraud Office has told Parliament's Justice committee that very few matters it takes on are not prosecuted.

The committee has released its report on the 2017/18 Annual review of the Serious Fraud Office. It says it was concerned about the relatively low number of prosecutions undertaken by the SFO in recent years, averaging below 10 per year.

Asked if this was a fair criticism, the SFO told the committee that the number of prosecutions depends on the size and complexity of the cases.

"It assured us that, while it needs to be selective, it does not reject many matters."

The report says that sometimes the SFO initially declines to prosecute but later, when more evidence comes forward, it pursues the matter.

"This happened with the Fuji Xerox case. The director said she was proud of the SFO’s ability to properly scope its investigations so that the work undertaken is focused and feasible."

Complaints not taken up by the SFO are referred to the most appropriate agency - most often the New Zealand Police, but also potentially to the Financial Markets Authority, the Commerce Commission and the Department of Internal Affairs.

The report says that the SFO was asked whether the small number of prosecutions was due to a lack of resources.

"The SFO said it does not dismiss investigations where staff believe there is a reasonable prospect of a successful prosecution. However, it acknowledged that sometimes delays occur due to the urgency or size of other cases.

"We noted that the number of complaints has increased by 20 percent to more than 1,000 a year. Given this trend, we asked whether the office has enough resources to give all complaints the proper attention. The SFO said that many of the complaints do not relate to its mandate of investigating serious or complex fraud. A large number are referred to other agencies as they do not meet the threshold, or do not involve fraud at all."