UPDATED: 8.06AM The Auckland mum who's taken a stand against sideline bullies at her son's rugby league game has won the praise of Sport New Zealand.

Rochelle Mara posted an open letter on Facebook over the weekend calling out parents who made fun of her 10-year-old son because of his weight and size.

Sport New Zealand's Geoff Barry said if we want kids to stay involved in sport it has to be about having fun but too often impassioned sideline spectators speak without thinking.

"The interaction with people on the sideline challenging that and putting them in a more aggressive mood, or challenging the way they look, or the way they're playing is clearly going to put them off. Is may put them off for life. We have to congratulate Mrs Mara for taking a stand and saying, it's just not necessary."

Mr Barry said there's never an excuse for sideline bullying, no matter how tense the game may be.

He told Jack Tame the move from encouraging comments to disparaging ones, can happen quickly in the heat of the moment.

"It is in the minority but unfortunately the negative element through verbal bullying, particularly verbal commentary, seems to be on the increase.

"Some critics might say it's parents living vicariously through their children but I really do think it is a lack of understanding."

He's commending Ms Mara for pointing it out - because drawing attention to it's the best way to stop it happening.

Clinical psychologist Karen Nimmo said verbal jostling at games isn't uncommon, but can discourage kids to keep playing.

"When you do hear repeated negative comments about various aspects of your personality and things that are hard to change, it can be off putting and it can make you retreat to the sidelines when we all know the healthy thing to do is be out there and participating."

Ms Nimmo said while it's disappointing to see such negativity in kids' sport, it isn't likely to go away anytime soon.

"People get really wound up and so they're always going to be out there in support of their team and their people, so I don't think we can stop that. What we have to be mindful of is negative comments can hurt, and a person's size is a fairly easy thing to attack."

Ms Nimmo said parents need to keep sticking up for their children and focus on being positive if kids are going to stick with sport.