The ACT government has rejected the standard method for delivering a kangaroo contraceptive vaccine as too damaging to the animals, but expects to find an alternative this year.
The fertility trial was announced two years ago amid community unease with the annual shooting of more than 1500 kangaroos around the city's nature parks.
Late last year, authorities began injecting kangaroos at trial sites with the Gonacon contraceptive, but to date the work has been done by hand.
The trial was designed to find a way to inject kangaroos by dart, so they can be vaccinated in sufficient numbers in the wild without having to tranquillise and inject by hand.
But senior ecologist in charge of the program Claire Wimpenny said the standard dart and dye had proved too heavy to deliver safely to kangaroos. With the contraceptive in one chamber and the marker dye in another, the vaccine had to be shot either at close range or at speed, and when shot at speed it hit with too much force.
The heavy darts fired at speed could cause "quite substantial tissue damage", she said.
The system had been trialled on ballistics gel, carcasses and targets and had now been rejected for the Canberra trial.
Instead, authorities were looking at ways to deliver the contraceptive and marker dye separately to each animal and she expected field trials would start this year.
Meantime, Gonacon has been hand-injected into 65 female kangaroos, with a placebo injected into another 10. Another 50 kangaroos had been collared ready for dart-injection later in the year.
Sites included Weston Creek, the Australian National Botanic Gardens and the Gold Creek golf club. Kangaroo numbers were also being monitored at other sites, including Lyneham Ridge and north Weston, to compare fertility in untreated populations.
Ms Wimpenny said the sites had been chosen for their small size and approachable groups of kangaroos, where they could be easily monitored. The process was complex and detailed, with a team of six people to tranquillise and vaccinate the animals, but so far it had gone smoothly.
One animal in the trial had died - a Weston Park kangaroo found it by a car on Hindmarsh Drive near Red Hill, a significant distance from Weston Park.
As to why authorities were hand-vaccinating kangaroos when the trial was to find a way of delivering it by dart, Ms Wimpenny said it was important to establish the effects of hand-vaccination also.
The only previous trial of Gonacon in eastern grey kangaroos was in 2008 in Canberra when 16 kangaroos at the Belconnen Naval Transmitting Station were treated and 10 injected with a placebo. That trial was on sub-adult kangaroos, yet to breed; the current trial was on breeding adults. Seven years later, one of the treated kangaroos was breeding; the others were still infertile.
The fertility trial was first announced almost two years ago.
In May 2014, then minister Shane Rattenbury announced a trial that was to involve 500 kangaroos in Gungahlin.
In March last year, Simon Corbell announced the trial would involve about 200 kangaroos and focus on dart-delivery. He said then that Gonacon was already known to be effective when administered by injection, but field trials were needed on dart-delivery.
The trial is being done in collaboration with the CSIRO.