They can't just flat out refuse you PT when your Dr is advising it - that is going against medical advice and is not in the best interests of the employer!! They can ask you to undergo a medical assessment where a doctor may review your history and then provide advice independent from the employer's and your own interests if that makes sense.
From an employer's perspective it may be a PITA to have someone go PT but they need to accomodate it especially if it is medical advice - to not follow it is not following their own duty of care in providing a safe work place.
Do you have a maternity leave/pregnancy policy? It may say something in there (or in your award/agreement) about managing 'illness' in pregnancy - I understand that technically you are not ill but this the term my employer would usually use to describe an employee needing a change to their employment circumstances based on
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