An estimated 26.9 million lambs were tailed, which was up 1.9 million on last year, Beef and Lamb New Zealand economic service's latest report shows.
Executive director Rob Davison attributed the increase to slightly more ewes mated (up 0.6%), favourable feed conditions before mating, and an increase in the number of lambs born from hoggets.
The average lambing percentage nationally was 123 lambs born per hundred ewes which was up from 119 in 2011.
While there were pockets of unfavourable weather in some areas during lambing, farm-management practices ensured good survival, Mr Davison said.
North Island lamb numbers were up 8.2% (990,000 head) to 13.13 million head, while South Island numbers were up 3% (398,000 head) to 13.74 million.
In Otago-Southland, the number of ewes to the ram increased 1.7% to 6.61 million, with the largest portion of the increase in Otago.
Lambing percentages increased 2.7% and 1.9% to 127.4% and 128% for Otago and Southland respectively.
The largest increase occurred on high-country farms, with a shift in farming practice on some properties towards crossbred ewes from merino.
Central Otago and Strath Taieri farms experienced excellent rainfall and pasture growth rates before mating which led to "exceptional" ewe condition at mating for the second consecutive season.
Hogget mating increased 7% in Otago-Southland.
The total number of lambs increased 6.3% to 4.58 million for Otago, and 2.4% to 4.26 million for Southland.
The service forecast there would be 20.5 million lambs available nationally for processing in the 2012-13 season - up an estimated 8.4%. That contrasted with last season's 18.9 million, the lowest since 1960-61.
The increase would be partially offset by an expected 2.1% decrease in average carcass weight to 18.3kg.
That followed a return to more normal climatic conditions after good growing conditions last season produced lambs that reached a record 18.7kg.
While there would be more lambs, there had been a sharp correction in lamb prices, with $5-$6/kg early in the season, compared to more than $8/kg in 2011.
A weakening outlook for schedule prices was creating uncertainty around early drafting decisions because livestock weight gains due to slow pasture growth rates might be outpaced by expected schedule declines, the report said.
Increased lamb numbers for more regions could mean some farms draft early if pasture growth rates became restricted, triggering early store sales on traditionally summer-dry properties.