A police officer with incurable cancer who swam across the Waitemata Harbour to raise money for others battling the disease is "rapt" with the generosity of an anonymous person who donated to $5000 to charity after reading her story in the Herald.

Detective Sarah Cato swam the 2.9km Harbour Crossing today to support Sweet Louise - a not-for-profit organisation that supports hundreds of Kiwis living with metastatic breast cancer.

She is fighting the disease herself, but hasn't let the diagnosis stop her from helping others and investigating serious crime - including the abduction and sexual assault of an 11-year-old Ranui boy a year ago.

She had a double mastectomy, but a scan later found the cancer had spread through her lymph nodes and blood into her bones.

Although Cato's cancer is incurable it is treatable and she is determined to keep fighting, going to the hospital every three weeks for a dose of Herceptin - a drug that targets metastatic breast cancer.

The Herald published a story earlier this week about the courageous 31-year-old and her goal do the Harbour Crossing.

After completing the race Cato told the Herald she was exhausted because she had to wake up at 5am for the race but was also stoked after finishing it in 1 hour and 21 minutes - 25 minutes faster than expected and "smashing" her $15,000 fundraising goal.

"It was such a beautiful morning. I actually had a little break in the middle of it and turned and looked at the harbour bridge, it was just lovely," she said.

"I probably could have swam a little bit longer. It was just amazing - that fitness level that you get after training in the pool. I think it will be an annual thing for me. I absolutely loved it."

The mother-of-one told the Herald that immediately after the story ran in Wednesday's paper someone made an anonymous $5000 donation to Sweet Louise via the charity's website.

"That's the most amount of money that they've had donated from a single donor since they've been running since 2004. I am just so proud," she said.

"I just couldn't believe it. What a generous person. That's just amazing. In my world that's a heck of a lot of money."

The donor probably didn't realise quite how valuable their contribution would have been to the families living with metastatic breast cancer who Sweet Louise supported.

"It gives families a chance to help them create these wonderful memories by allowing the families to spend their time on their kids and their grandchildren and with their friends and families rather than on chores. It just goes such a long way."

Another $12,200 had been raised for the charity through Cato's Everyday Hero fundraising page.

She celebrated her achievement by going out for breakfast with family, friends and police colleagues - including Waitemata district commander Superintendent Tusha Penny - who had cheered her on from the Viaduct Harbour finish line.

Cato said she felt privileged to have their support.

She planned to spend the rest of the weekend relaxing at home with her partner and 9-year-old daughter, but was looking forward to getting back in the ocean to swim again soon.

"I'm not much of a runner anymore. After the [cancer] treatment it's a little bit too hard on my bones doing other sorts of exercise, so swimming's perfect. That's how I'll stay fit and healthy."