"You had no reason to be there — you drove there for sexual gratification," Judge Smith said.
Wilkinson went into the public toilets, situated at the back of the main grandstand, where there was an 8-year-old boy using a urinal.
The defendant walked closely behind the victim then stood beside him.
The boy reported feeling the man's breath on the back of his neck, then his shorts were pulled down.
A witness heard the victim's vocal protests and went to his assistance while Wilkinson fled.
He walked away as people confronted him and was eventually chased down after making a sprint for his car.
Members of the public restrained Wilkinson as police were called to the scene.
The defendant initially denied the sexual assault but admitted being in the toilet.
Counsel Steve Turner said his client now fully accepted responsibility for the offending and was "disgusted with himself".
Wilkinson was motivated to do specialist sex-offender treatment, he said.
The judge said the incident was traumatising for the boy and his parents.
The ordeal for the "sensitive" victim "triggered a great deal of anxiety and stress", the judge said.
"[The parents] are very harsh on themselves, fearing they've let their boy down by not protecting him."
Wilkinson was jailed for 13-and-a-half months.
He was now subject to the restrictions of the Child Sex Offender Register.
Judge Smith warned Wilkinson if he committed further sex crimes he would likely find himself before the High Court.
Where to get help:
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334. (available 24/7)
• Better Blokes which provides peer support throughout Auckland, including a specific Pacific group.
• Male Survivors Aotearoa offers a range of confidential support at centres across New Zealand - find your closest one here.
• Mosaic - Tiaki Tangata: 0800 94 22 94 (available 11am - 8pm)
• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• https://www.lifeline.org.nz/services/suicide-crisis-helpline
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202
• NATIONAL ANXIETY 24 HR HELPLINE: 0800 269 4389
• If you have been abused, remember it's not your fault.