SYDNEY, Aug 16 (Reuters) - Hundreds of unarmed defence personnel joined thousands of extra police to set up roadblocks and enforce COVID-19 lockdown rules in parts of Sydney on Monday, while dozens of new venues were added to a list of exposure sites in Melbourne. Australia is in the grip of a third wave with its largest cities - Sydney and Melbourne - in lockdown along with the capital Canberra amid a slow vaccination rollout that has inoculated only 26% of people above 16 years of age. Tougher restrictions took effect in Sydney, the epicentre of Australia"s outbreak, after seven weeks of stay-home orders failed to stop the spread of the highly contagious Delta variant of the novel coronavirus. With the virus spreading into towns outside Sydney, the capital of New South Wales state, lockdown restrictions were imposed statewide for a week until Saturday night. read more In Melbourne, the state capital of Victoria, authorities detected 21 new locally acquired cases, down from 25 on Sunday as virus-exposed venues in the city topped 500. Australia said on Sunday it had bought about 1 million doses of Pfizer"s (PFE.N) COVID-19 vaccine from Poland with more than half rushed to inoculate the 20- to 39-year-olds in the worst-affected suburbs of Sydney. read more Prime Minister Scott Morrison downplayed suggestions that the United States did not volunteer to provide emergency supplies of coronavirus vaccines from its stockpile for Australia, one of its close allies. "We have an excellent relationship with the United States, but you have to focus on what is in front of you and what you need to do. That is what Sydney needed," Morrison told Nine News on Monday.