ABSTRACT Millimetre continuum observations of debris discs can provide insights into the physical and dynamical properties of the unseen planetesimals that these discs host. The material properties and collisional models of planetesimals leave their signature on the grain size distribution, which can be traced through the millimetre spectral index. We present 8.8 mm observations of the debris discs HD 48370, CPD-72 2713, HD 131488, and HD 32297 using the Australian Telescope Compact Array (ATCA) as part of the PLanetesimals Around TYpical Pre-main-seqUence Stars (PLATYPUS) survey. We detect all four targets with a characteristic beam size of 5 arcsec and derive a grain size distribution parameter that is consistent with collisional cascade models and theoretical predictions for parent planetesimal bodies where binding is dominated by self-gravity. We combine our sample with 19 other millimetre-wavelength-detected debris discs from the literature and calculate a weighted mean grain size power-law index that is close to analytical predictions for a classical steady-state collisional cascade model. We suggest the possibility of two distributions of q in our debris disc sample; a broad distribution (where q ∼ 3.2–3.7) for ‘typical’ debris discs (gas-poor/non-detection), and a narrow distribution (where q < 3.2) for bright gas-rich discs. Or alternatively, we suggest that there exists an observational bias between the grain size distribution parameter and absolute flux that may be attributed to the detection rates of faint debris discs at $\rm ∼$cm wavelengths.