The former New Zealand under-20 representative turned down a place in the All Blacks training squad last year, preferring to target England honours after joining Wasps at the end of his current Super Rugby campaign with the Wellington Hurricanes.
The 27-year-old, who is eligible for England courtesy of his parents, was not entitled to demand a release for this June's international window, New Zealand Rugby (NZR) said.
"Our contract agreement requires Brad to be available for only New Zealand teams until the end of his term, which expires at the end of this year's Super Rugby season," NZR's Chris Lendrum said in a statement.
"Having said that, Brad has been an exceptional leader inside the Hurricanes environment and a loyal servant to New Zealand Rugby.
"We have taken some time to carefully work through the complexities of this request, to ensure that we gave appropriate consideration to his specific circumstances. In the end, we are happy with the terms of his temporary release.
"We are extremely disappointed that the RFU chose to take this unusual step in seeking this release given that Brad has not yet played rugby in England."
A typically combative and mobile New Zealand loose forward, Shields won the 2016 Super Rugby title with the Hurricanes and helped them to a draw with the British and Irish Lions last year.
"I'm really appreciative that New Zealand Rugby has considered my request," the Hurricanes captain said.
"I know it’s been a complex matter for them but I’m hugely grateful for their support on this."
His potential selection for England while still playing in New Zealand has not been universally welcomed with World Cup winning coach Clive Woodward among those criticising the move.
Woodward said in his Daily Mail column this week that fast-tracking Shields into the squad was "intrinsically wrong" and a "slap in the face" for other English players who are unable to play for their country while contracted to foreign clubs.
Eddie Jones will name his squad for the three-test tour of South Africa later on Thursday.
