Millions of people have migrated between provinces in the past decade, the latest census reveals.

Gauteng has gained more than 3.1million people and Western Cape 1.3million, putting pressure on infrastructure and eroding the income base of poorer provinces.

Statistician-general Pali Lehohla said yesterday that Gauteng seemed to have a "pull effect".

The pull was money - or at least the opportunities it offered.

Gauteng has the richest population - more than 20000 people in the province earn more than R2.45-million a year. Another 30292 earn between R1.29-million and R2.45-million, according to the census.

Professor Carel van Aardt, who specialises in demographic and econometric modelling at the Bureau for Market Research, said the migrants were a mix of rich and poor. Two million people born in Limpopo, KwaZulu-Natal, Eastern Cape and Mpumalanga have settled in Gauteng since 2001.

"It is either the very poor that go to these areas for jobs, or the highly skilled and rich, who follow lifestyle and business opportunities," said Van Aardt.

With Gauteng's population growing by more than a third since 2001, it has overtaken KwaZulu-Natal as the most populous province, with 12.27million people.

Western Cape added nearly 29% to its population to 5.88million.

Almost 900000 people born in Eastern Cape have moved to Western Cape, boosting the proportion of Xhosa-speaking people in the province to nearly a third.

Earlier this year, Western Cape Premier Helen Zille - referring to the influx of people to her province - labelled pupils from Eastern Cape attending Western Cape schools "refugees" and compared the poorer province to an "incapacitated state".

As Gauteng and Western Cape - already the most prosperous provinces - grab a larger share of the population, they are also likely to get a bigger slice of the national government's budget.

According to Van Aardt, this will put the squeeze on poorer provinces' finances.

Municipalities in the poorer provinces will suffer even more as they continue to lose ratepayers to Western Cape and Gauteng's urban areas.

Howard Gabriels, chairman of the Statistics Council, which advises the government, said the migration figures were "surprising".

"What this suggests is that urbanisation in Gauteng and Western Cape is happening much faster than expected," he said.

He said it was important that the government align its financial planning with the new data.

"They need to set their programmes to deal with a broad range of service-delivery issues [and providing] access to basic services. With comprehensive new data set like this, it will definitely help the government to review its policies," he said.

Van Aardt said the census had confirmed some of his own research, which showed that large numbers of people had moved to urban centres, not only Johannesburg and Cape Town but also Durban and other cities.

Some of the metros, he said, gained more than others as entrepreneurs and professionals flocked to Gauteng's cities in search of bigger markets and other opportunities.

Professor Ivan Turok, deputy executive director of the Human Sciences Research Council, said economic data supported the census finding that Gauteng "has become the golden egg of the country".

Turok said internal migrants prompted many service-delivery protests and the government should take cognisance of this.

"There are service-delivery protests because people are moving and the services [they find] are not what they are expecting.

"Government funding does recognise this movement . the money is following the people with the services they need," he said.

President Jacob Zuma, who received the results of last year's census from Lehohla yesterday, said great strides had been made in improving the lives of South Africans.

Van Aardt agreed, saying the provision of housing, piped water and access to electricity contributed to the improvement.