Getting a sandwich in the city at lunchtime is taking a lot longer these days, with office towers near full capacity due to the national vacancy rate sitting at the lowest level for four years.

And given that the next wave of developments is at least three years away, particularly in Melbourne and Sydney, space is getting progressively tighter.

According to the latest data from JLL head of research Australia Andrew Ballantyne and head of office leasing Australia Tim O'Connor, the national CBD office market vacancy rate fell to 10.9 per cent in the three months to June 30, the lowest rate since March 2013.

In Melbourne the vacancy rate is 7.1 per cent, the lowest since March 2012, while Sydney tightened to 6.4 per cent, the lowest since June 2008.

Aside from improving white collar employment, the high demand for office space is being underpinned by the entrance of new tenants – the tech giants who want CBD addresses – and the rise of co-working firms.

With the corporatisation of the internet and social media enterprises, they all require head offices and to attract staff they need to be in relevant locations.

In the past two years, Linkedin, Twitter, Apple, Amazon Web Services, Atlassian and DropBox, among others, have leased CBD offices in Sydney and Melbourne and it is likely that Amazon also could look for space in the city.

In addition, the co-working companies, including WeWork, Servcorp and Wotso, are also leasing space that traditionally was the domain of banking, legal and financial services.

Another bonus for landlords is that the fall in space has bumped up rents.

Agents say a shortage of contiguous space options in the Melbourne CBD is exerting upward pressure on rents with prime gross effective rents increasing by 16.2 per cent over the 2017 financial year to an average $900 a square metre per annum. Sydney CBD prime gross effective rents increased by 25.3 per over the past financial year to an average $1300 a sqm p.a.

Mr Ballantyne said the second quarter was the first time that all six monitored CBD office markets recorded positive net absorption [net leasing] in the same quarter since September 2011.

"Leasing inquiry and activity has remained strong in Melbourne with the convergence between markets led by a sustained improvement in Brisbane and Perth. Net absorption is constrained in Sydney by a lack of contiguous stock and owners are experiencing high tenant retention rates and positive rental growth," he said.

Mr O'Connor said multiple industry sectors were contributing to positive leasing inquiry and activity in the Melbourne CBD. Technology firms and co-working operators were the growth sectors there with take-up from RocketSpace and Melbourne IT, while Dentsu Aegis Network leased expansion space at 643 Collins Street.

Mr O'Connor said only a limited number of assets could provide contiguous space in the Sydney CBD, resulting in higher tenant retention rates.

"Owners are capitalising on positive market sentiment and prime gross effective rental growth which is the strongest in the world." Mr O'Connor said.

Recently, the NSW government launched the Sydney Startup Hub, which leased 17,244 sqm at Wynyard Green, Sydney.

It was leased through Cushman & Wakefield. Head of office leasing at Cushman & Wakefield Tim Molchanoff and director Jamie King said alongside anchor tenants, not-for-profit co-working communities Fishburners, Stone & Chalk, The Studio and Tank Stream Labs and Jobs for NSW will continue to sub-lease to additional tenants that will occupy the space within the start-up hub.

The improving market conditions have also triggered a new wave of cashed-up investors to Australia.

According to the inaugural edition of Knight Frank's global research report, Active Capital: The Report 2017, Sydney is in the top 10 of global super cities for total volume of overseas investment in 2016, with $US5.7 billion of offshore funds snapping up property last year – placing it seventh globally. This was behind New York, central London, Paris, Berlin, Shanghai and Dublin.

Knight Frank senior director of research Jennelle Wilson said investors were increasingly searching the globe for yield in this low-return environment – and Sydney was expected to continue to attract offshore funds.

"Sydney has definitely been the focus of offshore investment, accounting for 45 per cent of inflows for 2016 and 2017 to date followed by Melbourne's 32 per cent and Brisbane's 10 per cent," Ms Wilson said.

To garner that demand, Singapore-based TrustCapital Advisors has started an official marketing process to sell five office assets along Australia's eastern seaboard in a move expected to raise about $700 million.

CBRE and JLL will steer the campaign for the properties, which are held across two private equity funds managed by TCA.

The assets – 150 Charlotte Street in Brisbane; 50 Pitt Street in Sydney; and 469 La Trobe Street, 850 Collins Street and 575 Bourke Street in Melbourne – are being sold in accordance with the funds' strategy and intended timeline.