It could be couple of things:
1. Battery and charging system
Its good to check the condition of the battery. If you are in NSW, you can take your battery to NRMA motorserve and they test it free of charge. It may need charging and they do this for free too. You may need to take the battery out for them though. I know NRMA battery roadside can also test battery drain and also alternator functionality. Be careful with the brand of the alternator if you need to replace it.

2. Starter
The brushes on the starter motor will wear and cause the starter motor to loose performance. Your mechanics will need to do this for you.