An extreme blue-green algae outbreak in Lake Burley Griffin's central basin forced the National Capital Authority to temporarily close the fountain on April 19. There were fears mist caused by recent strong winds could fling the nutrient-ridden fountain water to the shores.

A National Capital Authority spokeswoman said tests of two water samples taken this month returned algae levels low enough to turn the jet back on. But the authority decided to keep it offline until June to allow for final touches of its upgrade.

"In accordance with the ACT Guidelines for Recreational Water Quality, the NCA requires two consecutive weekly water samples below the benchmark level [of blue-green algae] to resume normal operations," the spokeswoman said.

"As part of the major refurbishment of the Captain Cook Memorial Jet, the National Capital Authority factored in a short shutdown period to inspect and complete the commissioning work on the national monument."

These works include a final inspection of tunnels, pipes, pumps and electrical systems.

During the outbreak, NCA chief executive Malcolm Snow urged the public to keep leaves and grass clippings away from gutters and drains to prevent nutrients from washing into the lake and causing algae outbreaks.

The latest hiccup came after it was hoped the fountain had moved on from its long string of setbacks.

Water had been spouting from the lake periodically since late last year, after almost two years of headaches forced the jet to be shut off.

But in March, the jet came back bigger and better than ever, shooting an extra five metres into the sky after the $3 million refurbishment.

The NCA spokeswoman said the current inspection was the last leg of works and that the national monument would be back in action next month.