Southern Cross Healthcare is New Zealand's largest health insurance provider, with more than 800,000 clients.
The not-for-profit company, employing about 1500 people, was established in 1961.
It runs 12 private hospitals and offers 11 healthcare plans, plus travel insurance.
Southern Cross now has an estimated 80 per cent share of the $500 million health insurance market, after last year winning a protracted legal battle to acquire Aetna Health.
It blames difficulties in integrating the two companies' computer systems for the delays in processing payouts.
Southern Cross sent members a letter explaining, and apologising for, the "unacceptable" delays.
"We ask, too, that unless you are experiencing hardship or unless absolutely necessary, you do not phone to follow up your claim.
"The claim backlog has meant many more people are phoning the Southern Cross 0800 number.
"So waiting queues on our phones are long and you may be asked to phone later."
In November, the company said premiums would rise early this year.
They rose by up to 30 per cent in February last year for the 311,600 policy-holders aged over 45.
The company says the average claim now costs an extra 7.6 per cent per member, or nearly $50.
Southern Cross clients face long wait on phone
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