Early signs indicate the South Island beech forests are set to produce a "mast year" in seeding - previously occurring in 2006, 2002, 2001 and 1999.
The highly variable seed offers a plentiful food source for rats and mice during winter, usually a time when food is scarce, causing rodent numbers to reach plague-like proportions in spring.
Department of Conservation scientist Graeme Elliott said the scale of this year's beech mast season was a one-in-10-year event and posed a significant threat for at-risk species.
The last significant mast year was in 1999, when entire populations of yellowheads (mohua) were destroyed in some areas, he said.
Fish and Game officer Bill Jarvie, of Te Anau, said while a mast season was bad news for endangered birds, it was likely to result in a bumper season come October 1 for anglers "after big fat brown trout".
As the mast - which can produce up to 18,000 seeds per square metre in some areas - began to decline, rodents often chose to swim across rivers and lakes seeking alternative food supplies.
Proving too much of a temptation, mice were snapped up whole by trout, which caused the fish to increase rapidly in size, Mr Jarvie said.
In 2002, a 3.5kg brown trout caught by a fellow angler on Lake Monowai revealed 13 whole mice from the contents of its stomach, he said.
"I wouldn't eat one but others don't mind.
''The flesh is quite pale as the fish has quite a bit of fat on it, so people often prefer to smoke them."
With trout capable of growing at a faster rate during a mast year, an angler's best chance to catch a large fish was to use a deer hair minnow and to fish at night, he said.
"These big trout bring out the punters."
