He has drawn his assistants from unlikely quarters.
"A couple of the Black Power Auckland chapter came and gave us a hand the first week we started. We've planted between 500 and 700 trees along here now," he says, scanning a length of the grassy bank.
"If it's any good we'll be able to plant right along here. I want to leave this place nice for my grandkids. I want to bring back the native birds," says Mr Diaz, a beneficiary and father of two.
Mr Tims helped him with the paperwork, securing plants and funds from the Nautilus Foundation, an Onehunga-based trust that raises funds for community, philanthropic and sports needs. Auckland Council chipped in, and more plants came from Alan Johnson, another local resident who's a Salvation Army policy analyst.
Mr Tims says Mr Diaz rallied people in the neighbourhood and has also been responsible for planting at the community house.
"He's developed a vege garden there, planted trees and flowers. He has a passion for horticulture and he's really good at getting the neighbours involved. At the end of the day, everyone's asking, 'Can we come back again?'.
"It always leads into more discussion about what else they can do in their community. One of the guys, who's been here for 10 years, says he's noticed in the last two years things seem to be a lot more positive around here."
Beyond the creek upgrade, the pair hopes to move on to improving their local park and sports fields, and making the whole area a safer, more welcoming place.
Mr Tims also sees a long-term win for Randwick Park and the chance to put the bad headlines behind them. If they can raise enough money, he'd like to be able to offer paid work to 10 people to help out, over a 15-week project, as their stepping-stone into employment.