Friends of the National Arboretum member Max Bourke pointed to a 2012 report to the government on the Molonglo development that said residents would have less green space than the Canberra average.

The report said future Molonglo residents would only have 2.4 hectares of green space per 1000 people, versus the then-Canberra average of 8.6 hectares per 1000 people.

The report also said there would be about 3300 people per square kilometre in Molonglo, compared to the then-Canberra average of about 1450 people per square kilometre.

Mr Bourke told the parliamentary inquiry into Canberra's green spaces that the capital was getting hotter and drier.

"Wedges of green are going to be more significant, particularly for the people who live in Molonglo," Mr Bourke said.

"The pub test in Canberra will have you understand that Canberra is getting more dense."

Speaking to The Canberra Times, Mr Bourke said Canberrans should watch for developers looking to slice off parts of the arboretum to accommodate the city's growth.

"A friend of mine said planning tends to let things look like there's an ice sculpture melting," Mr Bourke said.

The West Basin development was a good example, he said.

"[Developers] say 'We're only taking 2.4 per cent of the foreshore of the lake', but yeah, then someone else might take three per cent," Mr Bourke said.

"When you add up all the per cents that we're 'only' taking ... gradually these things melt away."

The 2012 report also said there would be about 3300 people per square kilometre in Molonglo, compared to the then-Canberra average of about 1450 people per square kilometre.

The submission said as the capital became more urbanised, the arboretum would become to Canberrans what Central Park was to New Yorkers.

Mr Bourke, along with a fellow member of Friends of the National Arboretum Dr Roger Hnatiuk, told the inquiry Molonglo residents might also need direct access to the arboretum.

The way it's designed now, residents would have to drive all the way around to Tuggeranong Parkway to get access.

But he said direct access from the west could be its own problem.

"Unless it's done properly, it'll become another rat run," Mr Bourke said.

Mr Bouke said when the arboretum originally opened it only predicted about 40,000 visitors per year, currently the arboretum sees about half-a-million visitors per year.

Dr Hnatiuk told Wednesday's inquiry that this unexpected growth had meant the resources were stretched.

He said most staff were using demountable offices. "That isn't working well," he said.

The submission called for more support and funding for the arboretum.