She learned the region had no weight restrictions on players over the age of 11, and the few that were in place were set so high as to be "farcical".
For example, despite being tall and well built for his age, her 14-year-old son has just passed the 60kg cut off mark for under-10 rugby.
But watching her son, at under-15 level, tackle one of the opposition's bigger players was "like watching a Mini taking on a bus", she said.
Mrs McNab believes the situation is leading to more injuries, and in particular concussions in young players.
The Herald's analysis of rugby-related ACC claims revealed a huge spike in injuries reported by 15- and 16-year-olds in 2013. ACC received 8611 injury claims from a playing pool of 18,098 15- and 16-year-olds - close to one claim for every two players throughout the country.
Mrs McNab said a decline in playing numbers among 15- and 16-year-olds could be directly linked to the injuries and lack of enjoyment created by mismatches. "Every year we lose more players because of the size mismatch and every year it is a little less safe for the ones who stay in the game.
"Eventually you have to bite the bullet and say, 'This can't go on. We have to do something about it'."
Since May, she has written repeatedly to the Wairarapa-Bush union, New Zealand Rugby, college principals and the Ministry of Education without receiving a satisfactory response.
She now plans to take her campaign to local schools' boards of trustees, questioning their duty of care.
NZ Rugby community rugby manager Brent Anderson said the issue would be on the agenda at a national meeting of provincial union delegates next month.
"We are very aware and conscious of the issues that are out there," Mr Anderson said.
"But, as many parents who write to us worried about big kids playing against little kids, we have an equal number of people saying, 'My big kid hasn't got a team to play in.' How fair is that?
"It is complex - there is no right answer - but it is in everyone's interest to make the game as safe as we can and that is what we look to do."
Wairarapa-Bush Rugby chief executive Tony Hargood said the union was focused on providing an enjoyable playing environment for as many children as possible but did not compromise on player safety.
"Our ultimate aim is have young players, both boys and girls, enjoying their Saturday morning rugby or Rippa and we try a put a lot of emphasis on participation," Mr Hargood said.
"Mixing teams from all areas - bear in mind we are a very rural region - provides challenges."
Lochore backs daughter's aims
New Zealand rugby legend Sir Brian Lochore is not involved in his daughter's campaign to improve safety measures in junior rugby, but he does support her aims.
Sir Brian Lochore said he agreed with his daughter that children were being placed in danger and scared away from the game. Photo / NZME.
Sandra McNab, Sir Brian's daughter, has spent months lobbying rugby and education officials over the dangers posed by size mismatches in junior rugby (see A12-13).
"What she has been doing is absolutely, totally independent of me," said Sir Brian, patron of New Zealand Rugby and a former All Black captain, selector and manager.
"I support her concerns, obviously, along with a lot of other parents."
Mrs McNab is petitioning New Zealand Rugby to introduce and enforce better controls governing the size of players following a string of incidents resulting in nasty injuries to young players in the Wairarapa.
Sir Brian said he agreed with his daughter that children were being placed in danger and scared away from the game.
"I do believe it is an issue all over New Zealand," he said.
"I've been along to a little bit of little kids' rugby to watch grandchildren. You'll get one or two big kids who, instead of running into the gap, will just go running into these little fellas because it is neat knocking them arse over head. That is the bit that really upsets the parents, I am sure."
Mrs McNab did not mention her father's identity when she approached the Herald with her concerns.
New Zealand Rugby's general manager of community rugby, Brent Anderson, a former All Black who earned 69 caps for Wairarapa-Bush, said rugby administrators faced a difficult job in balancing the needs of all children who wanted to play the game.
"The bigger unions with a big playing population can structure their grades to mitigate against big kids playing against small kids," Mr Anderson said.
Read more about parents' concern over safety in rugby here.