A durian tree can become acclimatised to some degree, but true climatic adaptation of the species to NSW conditions would likely require either:

Many generations of selective breeding, or
Discovery of naturally cold-tolerant genetic variants.
What a durian tree can acclimatise to

An individual durian tree can gradually adjust to:

Slightly cooler temperatures.
Seasonal changes in day length.
Lower humidity than tropical rainforests.
Temporary cold spells.

This is why trees grown from seedlings in NSW often tolerate conditions that would shock a freshly imported tropical plant.

However, acclimatisation has limits.

The main problem: cold damage

Durian evolved in the equatorial rainforests of Southeast Asia, particularly around regions such as Borneo and Sumatra.

The species generally performs best when:

Temperatures remain above about 20°C.
Humidity stays high.
Soil temperatures remain warm year-round.
Frost never occurs.

When temperatures fall into the single digits, several things happen:

Root activity slows dramatically.
Water uptake decreases.
Leaves may yellow and drop.
Growth can stop completely.
Prolonged cold can kill branches or the entire tree.

The leaf drop you've observed during NSW winters is a classic stress response.

Could NSW-grown durians become tougher over time?

Possibly.

Imagine planting 1,000 seedlings in NSW and allowing winter to naturally select the survivors.

After 20–30 years:

Some trees would prove more cold tolerant.
Seeds from those trees could be planted again.
The next generation would be selected from the toughest individuals.

Repeat this over multiple generations and eventually you might develop a "NSW strain" of durian that tolerates cooler conditions than its tropical ancestors.

This is essentially how many fruit crops became adapted to new climates throughout history.

What is more realistic?

Rather than adapting to all of NSW, durians may eventually become viable in specific microclimates such as:

Coastal Shoalhaven
Parts of the Illawarra
Protected north-facing slopes
Enclosed greenhouses
Large thermal-mass growing structures

These locations experience much milder winter conditions than inland NSW.

The root-zone theory

One of the more interesting possibilities is that soil temperature may matter nearly as much as air temperature.

If:

Air falls to 5–8°C overnight,
But root-zone temperatures remain around 18–22°C,

the tree may suffer far less stress than if both the air and soil become cold.

This aligns with your observations that winter protection and heating can dramatically improve survival.

A comparison

Think of citrus in Australia.

Early citrus originated in tropical and subtropical Asia. Through centuries of selection and cultivation, many varieties now tolerate conditions that would have killed their wild ancestors.

Durian could potentially follow a similar path, but because it grows slowly and takes years to fruit, the breeding process would be measured in decades rather than years.

My assessment

If someone asked whether durians will naturally become broad-acre commercial crops across NSW in the next decade, I would say no.

If they asked whether dedicated growers could, over several generations, develop a more cold-tolerant NSW-adapted durian population and successfully fruit trees in protected coastal microclimates, I would say that is entirely possible.

The first trees that consistently fruit in NSW may not be the most tropical-looking specimens. They may be the descendants of generations that survived winter after winter, slowly selecting for cold tolerance while retaining the characteristics that make durian unique.