Online job ads posted their biggest monthly decline in six years last month, in a further sign that Australia's recent economic weakness is about to feed through to higher unemployment.

Key points: An official measure of job ads is down 3.8 per cent over the past year, led by declines in property-related sectors

An official measure of job ads is down 3.8 per cent over the past year, led by declines in property-related sectors The NT, NSW and Queensland had the biggest falls, Tasmania the biggest rise in job ads

The NT, NSW and Queensland had the biggest falls, Tasmania the biggest rise in job ads Vacancies for university graduates rose, but jobs for less educated workers decreased

The Australian Government's official job vacancy report — which analyses ads on Seek, Career One and Australian Job Search — showed advertised positions fell 1.5 per cent in March in trend terms.

Last month it was a decline in demand for labourers, sales workers, administrative staff, technicians and tradies that led the decrease in positions advertised online.

Over the past year, a 14.9 per cent slump in job ads for machinery operators and drivers, a 13.2 per cent slide in ads for labourers and an 11.8 per cent decrease in vacancies for sales workers led an overall 3.8 per cent slide in positions being advertised.

The detailed numbers show a 21.3 per cent slump in ads for construction trades workers, a 19.6 per cent slide for construction and mining labourers, a 19.2 per cent slide in positions for architects and transport and design professionals, as well as a 12 per cent drop in jobs for sales representatives and agents.

These declines highlight the emerging fallout from the bust following Australia's biggest residential construction boom.

These falls were partly offset by a 5.3 per cent rise in ads for community and personal service workers, as well as a 3.1 per cent increase in demand for professionals.

In particular, ads surged 29.4 per cent for health and welfare support workers, 26.4 per cent for health diagnostic and therapy professionals, 23.2 per cent for medical practitioners and nurses and 20.2 per cent for education professionals.

"The National Disability Insurance Scheme (NDIS) and ageing population are creating skills shortages in the health services sector," wrote CommSec senior economist Ryan Felsman.

"The pace of digital change is supporting employer demand for information professionals."

Although, while vacancies for information professionals rose 9.3 per cent over the past year, demand for ICT professionals eased 0.2 per cent and ads for engineering, ICT and science technicians fell 9.8 per cent.

The department notes that the number of job ads remains 20.7 per cent (or 30,400 listings) higher than the level recorded five years ago.

However, the fall in job ads over the past year is concerning, as previous declines have historically correlated to increases in the unemployment rate.

Falling numbers of job advertisements tend to correlate with a rising unemployment rate. ( Supplied: Department of Jobs and Small Business )

The recent fall in job ads has been spread around the country, with only the ACT reporting an increase in vacant positions last month.

Over the past year, job ads fell most in the Northern Territory (10.5 per cent), New South Wales (6.2 per cent) and Queensland (4.3 per cent), with modest declines in South Australia and Victoria.

Western Australia had a modest increase in job ads, the ACT recorded a 3.4 per cent rise and Tasmania recorded a 19.5 per cent surge.

Less skilled workers bear brunt of jobs downturn

The department's data show strong trends as to who is bearing the brunt of the downturn in job vacancies, and it is overwhelmingly lower-skilled workers.

While job ads targeted at those with a university degree rose by 0.9 per cent over the past year and 34.7 per cent over the past five years, ads looking for people who have no more than a certificate I or high school education fell 12.7 per cent over the past 12 months and are down 10 per cent over the past five years.

Jobs targeted at those with vocational training or diplomas were down nearly 5 per cent over the past year but up more than 20 per cent over the past five.

Nearly 40 per cent of job ads online now require university qualifications.

However, Callam Pickering, an economist with global jobs website Indeed, which does not contribute to the department's figures, said that increased fragmentation in the job advertisement market may be affecting the data.

"The likely impact of this is that monthly figures are likely to be biased in a downward direction and that job advertisements are higher than reported by the Department for Jobs and Small Business," he told ABC News.