Through his parents he is immediately eligible, despite not ever playing professional rugby in England. Shields is moving to Wasps this summer, after his Super Rugby commitments are over, so will be a key figure in the Premiership.
SO WHY ARE THE KIWIS UPSET?
This row all started when reports up here claimed Shields was in line for England selection. Then Steve Tew, the CEO of New Zealand Rugby, questioned that - saying that they were under no obligation to release the player before his Hurricanes contract was over.
Because Super Rugby runs until August, they wanted to keep him in New Zealand.
'I don't think you can jump to the conclusion that he is available from our point of view,' Tew said in April.
'He is signed to New Zealand and he is contracted to play for New Zealand teams until the end of Super Rugby.'
They claimed that a contract clause, signed by all Super Rugby players - called 'Schedule Three' - precluded players from playing in anything other than Kiwi tournaments until their contracts were up.
It is not necessarily the case that they were annoyed that the RFU, and England, were nicking one of their players - after all, the All Blacks have had the chance to select Shields for years if they wanted to as he qualifies for them on residency but they did not. However, Shields did play in the 2011 U20s World Cup for the Baby Blacks against England alongside Beauden Barrett, Sam Cane and Brodie Retallick, and against Owen Farrell, George Ford, Joe Launchbury and Mako Vunipola - now international team-mates.
A cynic might say Tew and the NZR had to look strong in front of their public, as New Zealand is suffering something of a player-drain at the moment with many fringe Test players going to Europe in search of lucrative deals, but also that it was a chance to try and embarrass the RFU and make them look like they do not have a strong enough system so have to bring in 'foreign' players to compete.
DO THEY HAVE A POINT?
Not particularly - but yes to an extent. On the legal side, not particularly. Without going into tedious detail all rugby is governed by the laws and regulations set out by World Rugby - the sport's governing body. Everyone signs up to it and agrees that they have the final say on all matters.
In this case World Rugby - under 'Regulation 9' - demand that all players eligible for Test nations be released in two specific international windows; November and June. No other deals done by clubs, leagues or unions can trump that.
Adding to that the fact that NZR allowed Piers Francis - who was then at the Blues of Auckland - to tour Argentina with England and play in both Tests, without much fuss at all, it smacks a little of hypocrisy. The Shields case is pretty much identical, but NZR chose to make a fuss this time.
On the moral side the Kiwis do have a leg to stand on - to an extent. This whole episode has caused hot debate over whether England should be picking a man who is not from our system, has not played here, and is being fast-tracked into the World Cup plans as it seems like Eddie Jones has run out of options (the coach would dispute the latter). And when the likes of James Haskell seem to be on the way out, people do get frustrated.
AND WHAT IS ENGLAND'S STANCE?
England - and more particularly Jones - could not care less where a player comes from, as long as they qualify. Jones may change his mind on some things, but on the issue of picking foreign-born players, or those who have qualified on residency, he is always very clear.
Today he said: 'My responsibility is to pick England qualified players. I get a list and I pick the best out of that list. I don't decide the regulations.'
The likes of converted-Kiwis Ben Te'o, Jason Woodward, Willi Heinz and others have been picked in squads before.
And really you would say he is right. If players qualify, he can pick them. Whether it sits comfortably with you that the England head coach can sometimes disregard the carefully constructed 'pathways' to the England side - via junior teams, and the domestic league - is a different matter.
SO, CAN SHIELDS PLAY AGAINST SOUTH AFRICA?
Absolutely. NZR have reluctantly relented and eased their stance. On Wednesday night head of professional rugby Chris Lendrum said: '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.'
But he could not resist firing a shot at England.
'It seems highly unusual that they can't find players within their own county to pick,' he added.
Really, though, if it ever came to it World Rugby would intervene, show the Kiwis 'Regulation 9' and it would be over bar the shouting.
WHERE HAVE I SEEN HIM BEFORE?
Unless you are an avid Super Rugby fan - where Shields has been part of one of the best sides in that competition, the swashbuckling Hurricanes from Wellington, since 2012 - you would have last seen him in the 31-all draw between his Kiwi capital franchise and the British & Irish Lions. Shields had a fine game captaining the Hurricanes against Warren Gatland's men from No 8 in the midweek game, the one before the second Test in Wellington which the Lions won against the All Blacks.
WHAT ARE HIS STRENGTHS?
Shields is a hard, combative player who can play blindside flanker or No 8. He is not only a workhorse, but can carry ball and is skilful outside too. Think of a Chris Robshaw with slightly better hands and you are almost there. He is very fit too - he recently set a record at the Hurricanes in their 'Surf to Peak' challenge - a 1km sprint, 21km cycle and 2km run to the top of Mount Victoria in Wellington.
However, he has never played Test rugby - which is worlds away from Super Rugby - so it will be interesting to see how he adapts to a tighter game.
WILL HE TRAIN MUCH WITH ENGLAND BEFORE THE TOUR?
Not really. Like Francis did last summer Shields will just fly straight from New Zealand to South Africa to join up with England before the tour in early June. He will not come to England first or attend the national squad's pre-tour Brighton training camp. He is unlikely to feature in the first Test - as he will not be up to speed - but could well play in the next two.
AND WHAT WILL HE DO AFTER THE TOUR?
Shields - again, like Francis - will return to New Zealand, complete the Super Rugby season for the Hurricanes and then up sticks and move to Coventry, where he will link up with Wasps who will then be in the last throes of their pre-season before the 2018-19 season.
WHAT HAS EDDIE JONES SAID ABOUT HIS SELECTION?
After picking him the head coach said: 'Shields is a good hard working player who plays for the best team in the Southern Hemisphere. He is a considerable influence in their team, a team that has a lot of skill and talent. He's a hard working gritty player and a great back up to Chris Robshaw.'
COULD HE BE A WORLD-CUP BOLTER FOR ENGLAND?
Absolutely. If Shields can prove he is the combative, yet skilful, man to glue together England's troublesome back-row he definitely has a shot to secure a seat on the plane to Japan next year.
