Small health funds will be spared when Labor goes after the big private insurers and caps their annual premium increases.
Opposition Leader Bill Shorten told nib, Bupa and Medibank "the party's over" as he promised to cap premium rises at two per cent each year if Labor wins government.
But he also reassured smaller health funds they won't be unnecessarily targeted.
"We will listen to them, we will work with them to make sure there's no unintended consequences," Mr Shorten told reporters on Monday.
"The small funds are not the target of this issue."
A lobby group representing 23 smaller health funds met with leader Bill Shorten in June, when he provided them with "very strong reassurances" the cap would not be directed at them.
Members Health Fund Alliance chief executive Matthew Koce wrote a confidential email, published in The Australian on Monday, reassuring his members.
"The opposition leader and his office now appear to realise how devastating a two per cent cap would be to our sector and that it would perversely drive consolidation with the big, listed health insurers, who are the real target of the two per cent cap policy," he wrote.
The smaller funds mostly cover workers in regional areas and heavily unionised sectors such as nursing, teaching and mining.
Labor says its cap on premium increases will save families up to $340 on their private health insurance.
Prime Minister Malcolm Turnbull said Mr Shorten was doing "another one of his secret deals" for union-controlled health funds.
"Is he limbering up for yet another policy backflip or is it more like a belly flop?" Mr Turnbull told reporters in Melbourne.
Federal Health Minister Greg Hunt has unveiled a new health insurance rating system to make it easier for consumers to choose a fund.
The minimum hospital treatments covered by the new policy bands - Gold, Silver, Bronze and Basic - will come into place in April.
Mr Hunt says the system will not lead to a change in prices for procedures but allows consumers to easily see what their policies cover.
"For the first time, people will be able to see on a single page what is included and what isn't," he said.