Partners Life, which counts US private equity firm Blackstone as a cornerstone investor, posted a 13 per cent drop in annual profit as persistently low interest rates increased the life insurer's theoretical long-term liability, while its preferred underlying earnings measure was largely flat on the year.

Net profit fell to $9.7 million in the 12 months ended March 31 from $11.1m a year earlier, according to its annual report lodged with the Companies Office. That included an $8.5m charge from changes to economic assumptions reducing Partners Life's discount rate, $2.6m from investment revenue and a $3.1m gain from the release of an options reserve that wasn't needed.

Underlying earnings slipped to $12.5m from $12.6m, even as premium revenue rose 24 per cent to $171.5m, outpacing a 22 per cent increase in claims to $71m and a 7 per cent gain in operating costs to $42.2m. Customer numbers rose 15 per cent to 145,000, while in-force annual premiums were up 20 per cent to $207m.

"Our annual financial result has once again been underpinned by significant growth in our in-force book and a strong balance sheet," chief financial officer Sean Kam said in a statement. "We retained our leading new business market share position for risk business sold via independent financial advisers, and this was reflected in our premium income."

In May last year, Partners Group Holdings, the immediate parent of Partners Life, entered into a subscription agreement with Blackstone to invest $200m in three tranches over the next two years. Partners Life received the second tranche of $71.5m on June 28, adding to the $68.3m injection in September last year.

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The insurer's solvency margin rose to $73.5m from $25.6m a year earlier, giving it a solvency ratio of 150 per cent, up from 122 per cent at the end of the 2017 financial year.

The firm was launched in 2011 by Naomi Ballantyne, who was made an Officer of the New Zealand Order of Merit in the 2016 New Year Honours. Partners Life is the third life insurer she's had a hand in founding, having been one of the initial employees of Sovereign, now owned by ASB Bank, and ClubLife, now owned by ANZ Bank New Zealand as OnePath Life.

Partners Life paid $75.6m in commissions to advisers, up from $70.1m a year earlier, of which acquisition commissions were up 1.8 per cent to $58.1m and maintenance commissions climbed 31 per cent to $17.5m.

The company said it was "well placed to continue its strong momentum into the 2018 financial year" and led the market in adviser support.

Staff costs increased 5.2 per cent to $22.3m, while sales and marketing spending rose 16 per cent to $6.6m.