Objective: To assess changes in the choice of skin biopsy technique for assessing invasive melanoma in Victoria, and to examine the impact of partial biopsy technique on the accuracy of tumour microstaging. Design: Retrospective cross-sectional review of Victorian Cancer Registry data on invasive melanoma histologically diagnosed in Victoria during 2005, 2010, and 2015. Setting, participants: 400 patients randomly selected from each of the three years, stratified by final tumour thickness: 200 patients with thin melanoma (< 1.0 mm), 100 each with intermediate (1.0–4.0 mm) and thick melanoma (> 4.0 mm). Main outcome measures: Partial and excisional biopsies, as proportions of all skin biopsies; rates of tumour base transection and T-upstaging, and mean tumour thickness underestimation following partial biopsy. Results: 833 excisional and 337 partial diagnostic biopsies were undertaken. The proportion of partial biopsies increased from 20% of patients in 2005 to 36% in 2015 (P < 0.001); the proportion of shave biopsies increased from 9% in 2005 to 20% in 2015 (P < 0.001), with increasing rates among dermatologists and general practitioners. Ninety-four of 175 shave biopsies (54%) transected the tumour base; wide local excision subsequently identified residual melanoma in 65 of these cases (69%). Twenty-one tumours diagnosed by shave biopsy (12%) were T-upstaged. With base-transected shave biopsies, tumour thickness was underestimated by a mean 2.36 mm for thick, 0.48 mm for intermediate, and 0.07 mm for thin melanomas. Conclusion: Partial biopsy, particularly shave biopsy, was increasingly used for diagnosing invasive melanoma between 2005 and 2015. Shave biopsy has a high rate of base transection, reducing the accuracy of tumour staging, which is crucial for planning appropriate therapy, including definitive surgery and adjuvant therapy