Mr Hague said streams that are not covered by the policy are often places that are popular with local families.
"While they may be too small for adults to swim in, they are frequently the safe, slow rivers and streams that are perfect for children to splash about and play in."
Of New Zealand's 450,000km of waterways, about 54,000km of rivers and lakes - 12 per cent - are covered by the announcement.
Environment Minister Nick Smith clarified that the "90 per cent of rivers and lakes" he announced referred to what most people would reasonably call rivers and lakes - stronger flowing and larger - rather than 90 per cent of all waterways.
The rivers covered by the policy are those categorised as order four or above - larger rivers - and lakes with a perimeter of 1.5km or more.
"This national policy is not intended to answer every local and regional issue around water quality," Dr Smith said.
He said substandard feeding rivers or streams may need to be improved if they impacted swimmability of the larger rivers downstream.
"The vast bulk of our small streams are going to need to be improved if the river is," he said.
"Ninety per cent of those smaller streams are feeding into one of the water bodies that must be improved."
Forest and Bird said many feeding rivers could be polluted and not impact the quality of the larger parent river and are not directly covered by a standard.
The policy also excludes about 40km of streams nationwide which do not flow into a bigger river or lake. This included a number of urban streams.
Dr Smith said it was the regional council's responsibility to monitor and maintain the quality of rivers that weren't covered by the policy.
"Quite apart from the swimmability target, the national policy specifically requires that overall quality of freshwater must be improved and that legal requirement covers all water bodies.
"So there's a general requirement on regional council to improve the quality of fresh water in all of their fresh water management units - fourth order above and below."
"The issue for the public would be is if you want to insist on all of the streams and all of the ponds to be included, you'd substantially drive up their costs as ratepayers for monitoring small waterbodies and report on them when in our view the costs could not be justified.
"There may be quite sensible examples, like a small stream... that is popular and used and the regional council is absolutely entitled to, and should where it has need, to include that in its monitoring and report to its community."