Monday marks the start of the new season, and in its autumn outlook report, the Bureau of Meteorology said summer in 2020-21 was the wettest since 2016-17.

Maximum temperatures in the capital are likely to be below average between March and May, while minimum temperatures are likely to be slightly higher.

December last year was the third-wettest December since records began in 1900, and the bureau expects March to May will remain wet, although La Nina conditions are expected to end during Autumn.

While many Canberrans may have missed the heat, the summer rain was welcome at the National Arboretum after years of drought and last summer's choking smoke.

The institution was forced to close for four days last summer at the height of the smoke haze, but over the past year has transformed from a dust bowl to a COVID-safe oasis.

On average the arboretum used 1.5 million litres of water per week for irrigation during the peak of the drought.

"About 95 per cent of the trees are fully irrigated, so there's 44,000 drip tubes across the whole of the 100 forests of rare, endangered and symbolic species," National Arboretum and Stromlo Forest Park executive branch manager Scott Saddler said.

Between July 2020 and February 2021, the arboretum cut its use of irrigation almost in half compared with July 2019 to February 2020.

This reflects the vast differences between the two summer periods.

There are still signs of drought, but Mr Saddler warns some areas look worse than they are.

"There's a bit of a fallacy when people come to the arboretum and see a lot of dead grass. They think the National Arboretum is dead - but it's not," he said.

"I don't mind the dead grass because we're not irrigating the grass, we're only irrigating the trees."

Mr Saddler said while some trees, such as maples and Parana pines, bounced back after the drought broke, there were still "four or five forests" at the arboretum which still needed a "little more TLC".

Some trees, particularly species from higher-rainfall zones, struggled more than others during the drought and required additional water.

Weeping snow gums, Camden white gums, South Esk pines and Saharan and Moroccan cypress were particularly affected by drought and required additional hand-watering.

While irrigation was the key for the arboretum's survival last summer, due to wet weather from August 2020 through to early 2021 Mr Saddler said the arboretum rarely required irrigation over recent months.

Mr Saddler said he was confident the arboretum would be more resilient as trees matured.

"It's really difficult to look after the trees when they are in their infancy," he said.

"The older the trees get, the less moisture you need to grow that tree and the tree will find its own moisture.

"If I was just starting off here and we had trees across the whole arboretum that were 12 inches tall or two foot tall, I'd be worried.

"I'm not worried too much anymore."

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The arboretum can now access 200 megalitres of water each year from Lake Burley Griffin, to complement its 66-megalitre bore.

Water from Lake Burley Griffin wasn't available during the last drought, but will be made available in the event of another.

Mr Saddler expects the water from Lake Burley Griffin will drought-proof the arboretum for the next 30 years.

It wasn't just the trees that thrived at the arboretum in 2020. Visitor numbers grew by 20 per cent during 2020-21 compared to the 2019-20 summer period.

Unlike other national institutions, the arboretum was able to remain open during the lockdown - a decision that encouraged a spike in visitor numbers.

"I personally kept the National Arboretum open because this was Canberra's headspace," Mr Saddler said.

"They said build a National Arboretum and people will come - and that's what happened."

During the COVID-19 lockdown period the arboretum had a 28 per cent increase of visitors, with 700,000 people making the trip in the past 12 months.

"If you [came] to the arboretum during that COVID period, there were people riding their bikes and picnicking on the grass and walking on our tracks and trails," Mr Saddler said.

"The car park was full nearly every single day with people trying to get out of the house [but] be COVID-friendly."

Crowds didn't dwindle after COVID-19 restrictions eased.

"The bike riders from the western part of Canberra who ride through the National Arboretum to go to work in the city is exponential," Mr Saddler said.

"It makes me so joyous when I look out my window and see there's parents walking through the gallery of gardens, talking to their children, sitting on the grass and having picnics."

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