The manager of New Zealand's biggest fund has outlined the importance he places on beliefs when making investment decisions, while admitting that luck can play a big part.

Speaking at the Institute of Financial Advisers annual conference last week, New Zealand Superannuation Fund chief executive Adrian Orr described the unique challenges and opportunities involved with managing the fund, which is currently worth just over $18 billion.

He said the fund was in a "pretty rarefied, unique atmosphere" and should be able to "hit the ball out of the park" due to its ability to focus on the very long term (withdrawals don't begin until 2029).

Orr stressed the importance of beliefs for investment managers, saying: "Often when you're feeling the worst is when you're leaning the most on your beliefs."

However, he added: "Beliefs are just that; we won't know in 20 years' time whether any of our strategies worked because of our beliefs or whether it was because of luck."

KiwiSaver nears milestone

KiwiSaver has almost reached two million members but growth has slowed to a trickle, according to the latest monthly analysis by Tower.

"Net total KiwiSaver membership hit just short of 1.97 million by the end of June," said Tower Investments chief executive Sam Stubbs.

However, June's KiwiSaver growth rate was "somewhat sluggish" at just over 16,500 new members.

"The month was characterised by intensifying public and political debate over the merits of making KiwiSaver membership compulsory for working New Zealanders," Stubbs said.

"With the average monthly growth rate in membership much lower so far over 2012 than was seen the year before, it appears that most of those workers who would voluntarily embrace KiwiSaver have already done so.

"The question arises as to whether the remainder of working New Zealanders who are not KiwiSaver members should be required to sign up.

"Tower supports compulsory workplace superannuation saving and endorses KiwiSaver as the best way for most New Zealanders to save up for retirement."