She then began to see the towel moving up and down "vigorously".
She said she looked over again and tried to take in as much detail about him as possible as she thought she might need the information at a later date.
In questioning from defence counsel Glen Prentice about how many times she looked over, the victim said she looked about four or five times, but was wary about staring too much.
"I didn't because . . . it was very obvious he was watching me and probably wanted to be watched."
In his submissions to the judge, Prentice said given his client was seated 100m away there was a "clear issue of identity and what the witness saw".
He also said his client was covered the whole time and there needed to be consideration for whether the act was indecent.
However, Judge Clark was impressed with the witness' testimony stating that she "came across as clear with what she saw on the day".
Supporting the victim's testimony was Bell's interview with police on an unrelated matter when he admitted being on Kuaotunu beach in January and noticing a woman setting up a sunshade with a child.
"Having considered all the evidence and the supporting evidence from [police] I am satisfied that the person the [victim] saw was the defendant, Mr Bell."
The police case was that it his actions were also indecent and an inference could be drawn on his actions and the victim's testimony, and the judge agreed.
She was satisfied that Bell's actions would be considered indecent in the minds of right thinking members of the community.
"Having the heard the evidence which the victim gave quite carefully, was not making assumptions and was careful to confirm what she was seeing and what she was seeing him do was masturbating under his towel."
The judge said it was clear he was intending to carry out that act as he looked at the victim - who together with her son were the only people on the beach - as he carried out the act.
Prentice noted that Bell had been in custody since his arrest in early February and after Judge Clark convicted his client, urged he be sentenced as soon as possible.
Judge Clark agreed and remanded him in custody for sentencing on Monday.