I am travelling to New Zealand from India through Australia on 2 March and returning on 3 April. Do I have to apply for two transit visas, or will the visa I obtain for the outward trip on 2 March would do?
A single-entry or multiple-entry visa may be issued, depending on the needs and circumstances of the applicant and your proposed travel itinerary.
If you were already granted a visa, then as the about your visa page explains:
When you receive your Australian visa, you are issued with a visa grant notification letter that explains the conditions of your visa, including the period of validity and entry requirements. You should retain this for your own reference and you may wish to carry it with you when you travel as it contains important information about your visa. The information contained within the visa grant notification letter will help you to check your visa status online using our Visa Entitlement Verification Online (VEVO) system or myVEVO mobile app.
So go ahead, and check your visa, it will show whether it's for single or multiple travel.
If you have no visa yet then as you will file form 876: Application for Transit (subclass 771) visa for Australia you will see on "25. Arrival/Departure details" that you have space for three arrival-departure pairs which is plenty for you. I do not know whether you are allowed to continue on a separate page if you needed four or more.