Is a resignation due to unsafe working practices a constructive dismissal?

You have both a right and a responsibility to refuse unsafe work

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That is, an employer cannot take punitive action against an employee who refuses to perform a task on a good faith basis that the work itself is unsafe or that the work will create an unsafe situation. That’s the right.

In addition, a worker who knowingly performs work unsafely, or knowingly causes an unsafe situation has committed an offence. Albeit, these are typically only punished by the state in egregious circumstances. That’s the responsibility.

If there is a dispute about the safety of the work, it can and should be raised through the WHS channels that the employer was required by law to establish; these can be a WHS Committee, a WHS Representative, or any other suitable arrangements. If that fails to resolve the issue, the employer or the worker can request intervention from a Safe Work NSW inspector and each may be represented during that (paid for by the employer, if required).

Now, an employee who quits instead of going through that process, has probably not been constructively dismissed because there were administrative remedies that they did not pursue.

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The Employment Rights Act 1996, section 100 - Health and safety cases specifically excludes refusal to conduct unsafe work as valid grounds for dismissal.

If the employee left because of a danger "which the employee reasonably believed to be serious and imminent and which he could not reasonably have been expected to avert" then the employer has no grounds to dismiss.

However, if the employee is able to reduce the risk to an acceptable level, then walking out would not be protected, so there is scope for argument in establishing the severity of risk and what actions are reasonable as response.

So that's the case for unfair dismissal. However, this situation is different because the employee resigned, so we need to turn to section 95 - Circumstances in which an employee is dismissed and determine whether the employee resigned "in circumstances in which he is entitled to terminate it without notice by reason of the employer’s conduct". Refusal to implement reasonable safety measures (as the Health and Safety at Work Act requires) may be considered such conduct (but there may be argument as to what is reasonable depending on circumstances - Health and Safety at Work Act says that all "reasonably practicable" measures must be taken to ensure safety).