I don't write the law, but for the sake of answering, consent is implicit in this case. Sexual offences must be sexual in nature, and breastfeeding is not sexual. So any mother breastfeeding her child in public would be exempt from prosecution of such a sexual offence. Many traditional ceremonies are also conducted in traditional attire whereby the woman are topless. This is also not regarded as sexual and anyone attending such an event must implicitly consent here as well.

Our law makes many provisions for the reasonable man. So if you happen to stumble upon a nude beach (official status or otherwise), and be exposed to a female breasts, it would not be unlawful as people on this beach reasonably expected the people there to have consented. You could be charged with other offences such as public indecency if official nude beach status is not granted at that beach, but it would not be a sexual offence. Also, the prosecutors would likely be hesitant to waste the courts time on that, and drop the case. So the cops would likely just ask you to cover up if a complaint is received.

Our law is pretty good in that regards, it's our police and investigative side that is over worked and under skilled, mixed with loads of socio economic issues, that result in overloaded courts and high crime rates that we have.