MELBOURNE'S population has reached 4 million and Australia's is surging towards 22 million, according to new figures that have sparked fresh debate about the impact of record migration.

The Bureau of Statistics says Australia's annual net migration soared in the first three months of this year to 278,000 - up from just 100,000 five years ago.

Annual population growth was estimated at 439,000, almost double the level of five years ago. When the bureau's conservative methodology is taken into account, real annual population growth could be closer to 500,000.

At that rate, the nation may be only days away from reaching 22 million people - and Melbourne has probably passed 4 million already.

Victoria's population jumped 112,000 in the year to March. Assuming Melbourne has kept its share, the city is expanding by an unprecedented 90,000 people a year, or more than 1700 a week.