Melbourne’s suburban edge could expand to Wallan and Bacchus Marsh in the next 40 years, if the city’s middle suburbs were entirely protected from development.

A provocative map imagines what could happen if almost 4 million new residents forecast for the city by 2051 were to be shunted into the CBD centre and out to the far fringes.

Melbourne’s current growth boundaries would be breeched, with low-rise suburban sprawl rushing to meet country towns. In the heart of the city, more than 18,000 apartments would fit into an area the size of the Hoddle Grid.

The map is part of a series of visualisations dubbed “extreme”, even “propagandas”, by the Monash Architecture Studio researchers that created them.

But there is a point to the work. The architects want Melbournians to think about the consequences of failing to grow the city’s middle suburbs, and they have some challenging ideas about how this could be done.