An investment fund created to help cover the country's rising superannuation bill has reached a new record high, just shy of $29 billion.

The New Zealand Superannuation Fund hit headlines last month after revealing it had written off a loan to a Portuguese bank worth almost $200 million [http://www.stuff.co.nz/business/industries/66687224/nz-super-fund-very-confident-of-recovering-portuguese-loan].

While in the process of legal action to recover the taxpayer cash, the fund reported a 3.91 per cent return in February, taking its overall value to a new record high of $28.98b.

With an investment strategy tilted toward riskier growth assets, the fund's monthly performance can fluctuate up and down significantly.

"As a long-term investor we have a greater-than-average ability to withstand this volatility," the report said.

"Shifts in value from month to month must be seen in the context of the fund's long-term purpose and performance."

The fund was set up in 2003 to help offset the cost of a legion of baby boomers expected to retire from 2030 onward, but government contributions were suspended in 2009.

The fund has returned an average 10.3 per cent since inception, close to double the Treasury Bill rate, which represents the government's cost of contributing instead of paying down debt.

Earlier this year chief executive Adrian Orr warned the high returns of recent years were "the exception, not the rule", and unlikely to be repeated.

Over the long term the fund was expected to generate average returns of 8 per cent to 9 per cent a year, he said.

Future retirees' cash is primarily invested in North America and Europe, although 15 per cent is held in New Zealand assets.

Of those, the largest equity holdings are a 20 per cent stake in Z Energy worth $378 million, and a similar stake in Metlifecare worth $201m.

The fund's largest international stock exposure is a $172m slice of Apple, with other major holdings including Microsoft, Zurich Airport and Exxon Mobil.